Fix for the google namespace package on python2

we have to set the pep517 isolated build directory as the site dir for the google .pth file to work and find the nested protobuf package

sdks/python/setup.py

@@ -248,6 +248,14 @@ def generate_urn_files(log, out_dir):
  This is executed at build time rather than dynamically on import to ensure
  that it is compatible with static type checkers like mypy.
  """
+ if sys.version_info[0] == 2:
+ # this is a hack to make the google namespace package work on python2
+ import site
+ for path in sys.path:
+ if 'pep517-build-env' in path:
+ site.addsitedir(path)
+ break
+
  import google.protobuf.message as message
  import google.protobuf.pyext._message as pyext_message