Modernize all headers to Python 3 style (pantsbuild#7926)

Now that we require Python 3, we can simplify our header to remove `from __future__` and `# encoding=utf-8` entries, which both no-op in Python 3.

We use a [simple script](https://github.com/foursquare/fsqio/blob/4c8d43ad4140e42430f0b93dc9902e7851c24b7f/scripts/fsqio/python3-port-utils/pants/update_headers.py) to do this, along with updating `check_header.py` to only support Python 3 style headers.

build-support/bin/check_header.py

@@ -12,23 +12,13 @@
 from common import die
 
 
-EXPECTED_HEADER_PY2 = dedent("""\
-  # coding=utf-8
-  # Copyright YYYY Pants project contributors (see CONTRIBUTORS.md).
-  # Licensed under the Apache License, Version 2.0 (see LICENSE).
-
-  from __future__ import absolute_import, division, print_function, unicode_literals
-
-  """)
-
-EXPECTED_HEADER_PY3 = dedent("""\
+EXPECTED_HEADER = dedent("""\
   # Copyright YYYY Pants project contributors (see CONTRIBUTORS.md).
   # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
   """)
 
-EXPECTED_NUM_PY2_LINES = 6
-EXPECTED_NUM_PY3_LINES = 3
+EXPECTED_NUM_LINES = 3
 
 _current_year = str(datetime.datetime.now().year)
 _current_century_regex = re.compile(r'20\d\d')
@@ -47,10 +37,7 @@ def main() -> None:
     failures = '\n  '.join(str(failure) for failure in header_parse_failures)
     die(f"""\
 ERROR: All .py files other than __init__.py should start with the header:
-{EXPECTED_HEADER_PY3}
-
-If they must support Python 2 still, they should start with the header:
-{EXPECTED_HEADER_PY2}
+{EXPECTED_HEADER}
 
 ---
 
@@ -75,27 +62,17 @@ def check_header(filename: str, *, is_newly_created: bool = False) -> None:
   """Raises `HeaderCheckFailure` if the header doesn't match."""
   lines = get_header_lines(filename)
   check_header_present(filename, lines)
-  is_py3_file = all("from __future__" not in line for line in lines)
-  if is_py3_file:
-    lines = lines[:EXPECTED_NUM_PY3_LINES]
-    copyright_line_index = 0
-    expected_header = EXPECTED_HEADER_PY3
-  else:
-    copyright_line_index = 1
-    expected_header = EXPECTED_HEADER_PY2
   check_copyright_year(
-    filename, copyright_line=lines[copyright_line_index], is_newly_created=is_newly_created
-  )
-  check_matches_header(
-    filename, lines, expected_header=expected_header, copyright_line_index=copyright_line_index
+    filename, copyright_line=lines[0], is_newly_created=is_newly_created
   )
+  check_matches_header(filename, lines)
 
 
 def get_header_lines(filename: str) -> List[str]:
   try:
     with open(filename, 'r') as f:
       # We grab an extra line in case there is a shebang.
-      lines = [f.readline() for _ in range(0, EXPECTED_NUM_PY2_LINES + 1)]
+      lines = [f.readline() for _ in range(0, EXPECTED_NUM_LINES + 1)]
   except IOError as e:
     raise HeaderCheckFailure(f"{filename}: error while reading input ({e})")
   # If a shebang line is included, remove it. Otherwise, we will have conservatively grabbed
@@ -106,7 +83,7 @@ def get_header_lines(filename: str) -> List[str]:
 
 def check_header_present(filename: str, lines: List[str]) -> None:
   num_nonempty_lines = len([line for line in lines if line])
-  if num_nonempty_lines < EXPECTED_NUM_PY3_LINES:
+  if num_nonempty_lines < EXPECTED_NUM_LINES:
     raise HeaderCheckFailure(f"{filename}: missing the expected header")
 
 
@@ -123,21 +100,19 @@ def check_copyright_year(filename: str, *, copyright_line: str, is_newly_created
     )
 
 
-def check_matches_header(
-  filename: str, lines: List[str], *, expected_header: str, copyright_line_index: int
-) -> None:
-  copyright_line = lines[copyright_line_index]
+def check_matches_header(filename: str, lines: List[str]) -> None:
+  copyright_line = lines[0]
   sanitized_lines = lines.copy()
-  sanitized_lines[copyright_line_index] = "# Copyright YYYY" + copyright_line[16:]
-  if "".join(sanitized_lines) != expected_header:
+  sanitized_lines[0] = "# Copyright YYYY" + copyright_line[16:]
+  if "".join(sanitized_lines) != EXPECTED_HEADER:
     raise HeaderCheckFailure(f"{filename}: header does not match the expected header")
 
 
 def check_dir(directory: str, newly_created_files: Iterable[str]) -> List[HeaderCheckFailure]:
   header_parse_failures: List[HeaderCheckFailure] = []
   for root, dirs, files in os.walk(directory):
     for f in files:
-      if not f.endswith('.py') or os.path.basename(f) == '__init__.py':
+      if not f.endswith('.py') or os.path.basename(f) == '__init__.py' or root.endswith('contrib/python/checks/checker'):
         continue
       filename = os.path.join(root, f)
       try:

build-support/travis/generate_travis_yml.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import pkg_resources
 import pystache
 

build-support/travis/generate_travis_yml_main.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from generate_travis_yml import generate_travis_yml
 
 

contrib/avro/src/python/pants/contrib/avro/register.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/avro/src/python/pants/contrib/avro/targets/java_avro_library.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.backend.jvm.targets.jvm_target import JvmTarget
 from pants.base.payload import Payload
 

contrib/avro/src/python/pants/contrib/avro/tasks/avro_gen.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 
 from pants.backend.jvm.targets.java_library import JavaLibrary

contrib/avro/tests/python/pants_test/contrib/avro/tasks/test_avro_gen.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from textwrap import dedent
 
 from pants.build_graph.build_file_aliases import BuildFileAliases

contrib/avro/tests/python/pants_test/contrib/avro/tasks/test_avro_gen_integration.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/examples/hello_handler.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import pycountry
 
 from pants.contrib.awslambda.python.examples.hello_lib import say_hello

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/examples/hello_lib.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 
 def say_hello(s):
   print('Hello {}!'.format(s))

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/register.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.build_graph.build_file_aliases import BuildFileAliases
 from pants.goal.task_registrar import TaskRegistrar as task
 

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/subsystems/lambdex.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.backend.python.subsystems.python_tool_base import PythonToolBase
 
 

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/targets/python_awslambda.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.backend.python.targets.python_binary import PythonBinary
 from pants.base.exceptions import TargetDefinitionException
 from pants.base.payload import Payload

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/tasks/lambdex_prep.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.backend.python.tasks.python_tool_prep_base import PythonToolInstance, PythonToolPrepBase
 
 from pants.contrib.awslambda.python.subsystems.lambdex import Lambdex

contrib/awslambda/python/src/python/pants/contrib/awslambda/python/tasks/lambdex_run.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import functools
 import os
 import shutil

contrib/awslambda/python/tests/python/pants_test/contrib/awslambda/python/test_python_awslambda_integration.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 
 from pants.util.contextutil import temporary_dir

contrib/buildgen/src/python/pants/contrib/buildgen/build_file_manipulator.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import ast
 import logging
 import re

contrib/buildgen/tests/python/pants_test/contrib/buildgen/test_build_file_manipulator.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from builtins import open
 from textwrap import dedent
 

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/buildozer.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import logging
 
 from pants.base.build_environment import get_buildroot

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/buildozer_binary.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import logging
 
 from pants.base.build_environment import get_buildroot

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/meta_rename.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import logging
 from collections import defaultdict
 

contrib/buildrefactor/src/python/pants/contrib/buildrefactor/register.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.buildrefactor.buildozer import Buildozer

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/buildozer_util.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 
 def prepare_dependencies(self):
   self.add_to_build_file('a', 'java_library(name="a")')

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_buildozer.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 import re
 from builtins import open

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_buildozer_integration.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
 
 

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_meta_rename.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 
 from pants.backend.jvm.targets.java_library import JavaLibrary

contrib/buildrefactor/tests/python/pants_test/contrib/buildrefactor/test_meta_rename_integration.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants_test.pants_run_integration_test import PantsRunIntegrationTest
 
 

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/register.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 from pants.goal.task_registrar import TaskRegistrar as task
 
 from pants.contrib.codeanalysis.tasks.bundle_entries import BundleEntries

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/tasks/bundle_entries.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 import shutil
 

contrib/codeanalysis/src/python/pants/contrib/codeanalysis/tasks/extract_java.py

@@ -1,9 +1,6 @@
-# coding=utf-8
 # Copyright 2017 Pants project contributors (see CONTRIBUTORS.md).
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-from __future__ import absolute_import, division, print_function, unicode_literals
-
 import os
 from builtins import next