Use correct suffix for Python C extensions (RobotLocomotion#14730)

tools/skylark/pybind.bzl

@@ -2,6 +2,7 @@
 
 load("//tools/skylark:py.bzl", "py_library")
 load("@cc//:compiler.bzl", "COMPILER_ID")
+load("@python//:version.bzl", "PYTHON_EXTENSION_SUFFIX")
 
 # @see bazelbuild/bazel#3493 for needing `@drake//` when loading `install`.
 load("@drake//tools/install:install.bzl", "install")
@@ -54,7 +55,7 @@ def pybind_py_library(
 
     # TODO(eric.cousineau): See if we can keep non-`*.so` target name, but
     # output a *.so, so that the target name is similar to what is provided.
-    cc_so_target = cc_so_name + ".so"
+    cc_so_target = cc_so_name + PYTHON_EXTENSION_SUFFIX
 
     # GCC and Clang don't always agree / succeed when inferring storage
     # duration (#9600). Workaround it for now.

tools/workspace/lcm/package.BUILD.bazel

@@ -14,6 +14,7 @@ load(
     "install_files",
 )
 load("@drake//tools/lint:python_lint.bzl", "python_lint")
+load("@python//:version.bzl", "PYTHON_EXTENSION_SUFFIX")
 
 licenses([
     "notice",  # BSD-3-Clause
@@ -166,7 +167,7 @@ cc_binary(
 )
 
 cc_binary(
-    name = "_lcm.so",
+    name = "_lcm" + PYTHON_EXTENSION_SUFFIX,
     srcs = [
         "lcm-python/module.c",
         "lcm-python/pyeventlog.c",
@@ -216,19 +217,20 @@ generate_file(
     content = """
 import ctypes
 import os.path
-ctypes.cdll.LoadLibrary(os.path.realpath(__path__[0] + '/_lcm.so'))
+ctypes.cdll.LoadLibrary(os.path.realpath(
+     __path__[0] + '/_lcm{extension_suffix}'))
 _filename = __path__[0] + \"/lcm-python/lcm/__init__.py\"
 with open(_filename) as f:
     _code = compile(f.read(), _filename, 'exec')
     exec(_code)
-""",
+""".format(extension_suffix = PYTHON_EXTENSION_SUFFIX),
     visibility = ["//visibility:private"],
 )
 
 py_library(
     name = "lcm-python-upstream",
     srcs = ["lcm-python/lcm/__init__.py"],  # Actual code from upstream.
-    data = [":_lcm.so"],
+    data = [":_lcm" + PYTHON_EXTENSION_SUFFIX],
     visibility = ["//visibility:private"],
 )
 
@@ -293,7 +295,7 @@ install_files(
 install(
     name = "install_python",
     targets = [
-        ":_lcm.so",
+        ":_lcm" + PYTHON_EXTENSION_SUFFIX,
         ":lcm-python-upstream",
     ],
     library_dest = "@PYTHON_SITE_PACKAGES@/lcm",

tools/workspace/python/repository.bzl

@@ -149,6 +149,11 @@ def _impl(repository_ctx):
     # similar to that used in pkg_config.bzl and should be refactored and
     # shared instead of being duplicated in both places.
 
+    extension_suffix = execute_or_fail(
+        repository_ctx,
+        [py_info.python_config, "--extension-suffix"],
+    ).stdout.strip()
+
     includes = []
 
     for cflag in cflags:
@@ -199,10 +204,12 @@ def _impl(repository_ctx):
 # `BUILD.bazel` or `package.BUILD.bazel` files.
 
 PYTHON_BIN_PATH = "{bin_path}"
+PYTHON_EXTENSION_SUFFIX = "{extension_suffix}"
 PYTHON_VERSION = "{version}"
 PYTHON_SITE_PACKAGES_RELPATH = "{site_packages_relpath}"
 """.format(
         bin_path = py_info.python,
+        extension_suffix = extension_suffix,
         version = py_info.version,
         site_packages_relpath = py_info.site_packages_relpath,
     )