Fix usage of assertItemsEqual on Py3

Signed-off-by: Dan O'Reilly <oreilldf@gmail.com>

python/google/protobuf/internal/generator_test.py

@@ -291,9 +291,9 @@ class GeneratorTest(unittest.TestCase):
     self.assertIs(desc.oneofs[0], desc.oneofs_by_name['oneof_field'])
     nested_names = set(['oneof_uint32', 'oneof_nested_message',
                         'oneof_string', 'oneof_bytes'])
-    self.assertItemsEqual(
+    self.assertEqual(
         nested_names,
-        [field.name for field in desc.oneofs[0].fields])
+        set([field.name for field in desc.oneofs[0].fields]))
     for field_name, field_desc in desc.fields_by_name.items():
       if field_name in nested_names:
         self.assertIs(desc.oneofs[0], field_desc.containing_oneof)