Fix a six.with_metaclass usage mistake

I made this mistake in 2010985ab2 but
only with yesterday's release of six 1.11.0 has it started failing
("TypeError: metaclass conflict: the metaclassof a derived class must
be a (non-strict) subclass of the metaclasses of all its bases").

src/python/grpcio_testing/grpc_testing/__init__.py

@@ -213,7 +213,7 @@ class StreamStreamChannelRpc(six.with_metaclass(abc.ABCMeta)):
         raise NotImplementedError()
 
 
-class Channel(six.with_metaclass(abc.ABCMeta), grpc.Channel):
+class Channel(six.with_metaclass(abc.ABCMeta, grpc.Channel)):
     """A grpc.Channel double with which to test a system that invokes RPCs."""
 
     @abc.abstractmethod