Add version check for Python 3.6 to simple stubs

src/python/grpcio/grpc/experimental/__init__.py

@@ -85,6 +85,6 @@ __all__ = (
     'insecure_channel_credentials',
 )
 
-if sys.version_info[0] >= 3:
+if sys.version_info[0] == 3 and sys.version_info[1] >= 6:
     from grpc._simple_stubs import unary_unary, unary_stream, stream_unary, stream_stream
     __all__ = __all__ + (unary_unary, unary_stream, stream_unary, stream_stream)