Replace not_called assert statement with assert_not_called()

Resolves error during test:

AttributeError: 'not_called' is not a valid assertion. Use a spec for the mock if 'not_called' is meant to be an attribute.

test/service/test_manifest_async.py

@@ -336,9 +336,9 @@ def test(self,
                         self.assertNotEqual(token_url, response.json()['Location'])
 
             assert signed_manifest_key.encode() == manifest_url.path.segments[-1]
-            assert verify_manifest_key.not_called
+            verify_manifest_key.assert_not_called()
             verify_manifest_key.return_value = manifest_key
-            assert get_cached_manifest_with_key.not_called
+            get_cached_manifest_with_key.assert_not_called()
             get_cached_manifest_with_key.return_value = manifest
             response = requests.get(str(manifest_url), allow_redirects=False)
             self.assertEqual(302, response.status_code)