Partially implement gRPC Python server reflection

Some features relating to proto2 extension querying are missing due to
missing upstream features required to support them (see
https://github.com/google/protobuf/issues/2248).
diff --git a/src/python/.gitignore b/src/python/.gitignore
index f158efa..7b52057 100644
--- a/src/python/.gitignore
+++ b/src/python/.gitignore
@@ -1 +1,3 @@
 gens/
+*_pb2.py
+*_pb2_grpc.py