Add a setuptools extension for generating Python protobufs (#7783)

This extension allows Python code to be generated from setup.py files, so they
are created as part of a normal Python build. The extension uses an already-existing
protoc binary, which can be explicitly specified if needed.
diff --git a/python/setup.py b/python/setup.py
index d29ff22..b44e88b 100755
--- a/python/setup.py
+++ b/python/setup.py
@@ -284,6 +284,7 @@
       packages=find_packages(
           exclude=[
               'import_test_package',
+              'protobuf_distutils',
           ],
       ),
       test_suite='google.protobuf.internal',