Add __bool__ as well as __nonzero__ for python3

Author: anandolee

python/google/protobuf/internal/message_test.py

@@ -166,6 +166,9 @@ class BadArg(object):
       def __nonzero__(self):
         raise BadArgError()
 
+      def __bool__(self):
+        raise BadArgError()
+
     with self.assertRaises(BadArgError):
       golden_message.SerializeToString(deterministic=BadArg())