fix test_message_serializer.py (#1061)

* fix test_message_serializer.py

* use version_info etc

* flake8 wants a blank line

Author: Matt Howlett

tests/avro/test_message_serializer.py

@@ -23,12 +23,16 @@
 import struct
 
 import unittest
-from unittest.mock import patch
+from sys import version_info
+
+if version_info >= (3,):
+    from unittest.mock import patch
 
 from tests.avro import data_gen
 from confluent_kafka.avro.serializer.message_serializer import MessageSerializer
 from tests.avro.mock_schema_registry_client import MockSchemaRegistryClient
 from confluent_kafka import avro
+from pyflakes.test.harness import skipIf
 
 
 class TestMessageSerializer(unittest.TestCase):
@@ -76,6 +80,8 @@ def test_encode_record_with_schema(self):
             message = self.ms.encode_record_with_schema(topic, basic, record)
             self.assertMessageIsSame(message, record, schema_id)
 
+    @skipIf(version_info < (3,),
+            'unittest.mock.patch not available in Python 2')
     def test_encode_record_with_schema_sets_writers_cache_once(self):
         topic = 'test'
         basic = avro.loads(data_gen.BASIC_SCHEMA)