Skip setUpClass which is newly added in python2.7 for python2.6

Author: anandolee

python/google/protobuf/internal/message_test.py

@@ -1832,8 +1832,9 @@ def testUnpackedFields(self):
     self.assertEqual(golden_data, message.SerializeToString())
 
 
-@unittest.skipIf(api_implementation.Type() != 'cpp',
-                 'explicit tests of the C++ implementation')
+@unittest.skipIf(api_implementation.Type() != 'cpp' or
+                 sys.version_info < (2, 7),
+                 'explicit tests of the C++ implementation for PY27 and above')
 class OversizeProtosTest(BaseTestCase):
 
   @classmethod