workaround for missing 'assert_called_once' method

this method was introduced in Python 3.6, but calling it still
works (i.e. it doesn't cause the test to fail) on Python 3.3/3.4

test/test_cookies.py

@@ -69,7 +69,7 @@ class TestCookiejar(unittest.TestCase):
         with mock.patch.object(log, "warning") as mock_warning:
             cookies = extractor.find("test:").session.cookies
             self.assertEqual(len(cookies), 0)
-            mock_warning.assert_called_once()
+            self.assertEqual(mock_warning.call_count, 1)
             self.assertEqual(mock_warning.call_args[0][0], "cookies: %s")
             self.assertIsInstance(mock_warning.call_args[0][1], exc)