Further maintenance of Python dependencies

These changes probably should have been in 3b0fefb246caf9cf983d8 but
were overlooked.
diff --git a/src/python/grpcio_test/setup.py b/src/python/grpcio_test/setup.py
index 216119f..fe36bc9 100644
--- a/src/python/grpcio_test/setup.py
+++ b/src/python/grpcio_test/setup.py
@@ -72,6 +72,8 @@
 _INSTALL_REQUIRES = (
     'oauth2client>=1.4.7',
     'grpcio>=0.11.0b0',
+    # TODO(issue 3321): Unpin protobuf dependency.
+    'protobuf==3.0.0a3',
 )
 
 _COMMAND_CLASS = {