Remove references to six (protocolbuffers#9096)

We no longer support Python 2 and therefore no longer need to depend on
six. I'm hoping this will make it possible to merge protocolbuffers#8214.

BUILD

@@ -954,7 +954,6 @@ py_proto_library(
     protoc = ":protoc",
     py_libs = [
         ":python_srcs",
-        "@six//:six",
     ],
     srcs_version = "PY2AND3",
     visibility = ["//visibility:public"],

Makefile.am

@@ -1426,7 +1426,6 @@ EXTRA_DIST = $(@DIST_LANG@_EXTRA_DIST)   \
   examples/pubspec.yaml                  \
   protobuf.bzl                           \
   protobuf_deps.bzl                      \
-  third_party/six.BUILD                  \
   third_party/zlib.BUILD                 \
   util/python/BUILD                      \
   internal.bzl

kokoro/linux/aarch64/python_run_tests_with_qemu_aarch64.sh

@@ -7,7 +7,7 @@ cd $(dirname $0)/../../..
 cd python
 
 PYTHON="/opt/python/cp38-cp38/bin/python"
-${PYTHON} -m pip install --user six pytest auditwheel
+${PYTHON} -m pip install --user pytest auditwheel
 
 # check that we are really using aarch64 python
 (${PYTHON} -c 'import sysconfig; print(sysconfig.get_platform())' | grep -q "linux-aarch64") || (echo "Wrong python platform, needs to be aarch64 python."; exit 1)

kokoro/release/python/windows/build_artifacts.bat

@@ -9,7 +9,6 @@ set PACKAGE_NAME=protobuf
 set REPO_DIR=protobuf
 set BUILD_DLL=OFF
 set UNICODE=ON
-set PB_TEST_DEP="six==1.9"
 set OTHER_TEST_DEP="setuptools==38.5.1"
 set OLD_PATH=C:\Program Files (x86)\MSBuild\14.0\bin\;%PATH%
 

protobuf_deps.bzl

@@ -24,14 +24,6 @@ def protobuf_deps():
             urls = ["https://github.com/madler/zlib/archive/v1.2.11.tar.gz"],
         )
 
-    if not native.existing_rule("six"):
-        http_archive(
-            name = "six",
-            build_file = "@com_google_protobuf//:third_party/six.BUILD",
-            sha256 = "d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73",
-            urls = ["https://pypi.python.org/packages/source/s/six/six-1.12.0.tar.gz"],
-        )
-
     if not native.existing_rule("rules_cc"):
         http_archive(
             name = "rules_cc",

third_party/BUILD

@@ -1 +1 @@
-exports_files(["six.BUILD", "zlib.BUILD"])
+exports_files(["zlib.BUILD"])