Inconsistency between bazel and setuptools builds of CPython extension module.

[mitchallain]

The CPython extension module build configuration should provide extra_compile_args=["-fvisibility=hidden"] to be equivalent to the Bazel build and to prevent upb symbols leaking into the public interface of the Python extension module. This can create segfaults for applications linking against libprotobuf.so which have duplicate upb symbols and duplicate extension registries

protobuf/python/dist/setup.py

Lines 82 to 89 in 58f02be

	ext_modules=[
	Extension(
	'google._upb._message',
	srcs,
	include_dirs=[current_dir, os.path.join(current_dir, 'utf8_range')],
	language='c',
	extra_link_args=extra_link_args,
	)

The Bazel configuration (note copts):

protobuf/python/py_extension.bzl

Lines 16 to 27 in 58f02be

	cc_binary(
	name = name + "_binary",
	srcs = srcs,
	copts = copts + ["-fvisibility=hidden"],
	linkopts = selects.with_or({
	(
	"//python/dist:osx_x86_64",
	"//python/dist:osx_aarch64",
	): ["-Wl,-undefined,dynamic_lookup"],
	"//python/dist:windows_x86_32": ["-static-libgcc"],
	"//conditions:default": [],
	}),