message: Print protobuf type stubs warning to stderr
  output printed to stdout needs to follow the protobuf format
diff: @@ -22,7 +22,7 @@ try:
  import six
  from google.protobuf.compiler import plugin_pb2 as plugin # type: ignore
  except ImportError as e:
  - print('Failed to generate mypy stubs: {}'.format(e))
  + sys.stderr.write('Failed to generate mypy stubs: {}\n'.format(e))
  sys.exit(0)
  MYPY = False
  