Replace equals None with is None

None is a singleton and should be tested for with is not ==.

Some people did a similar thing randomly on the master branch. It
just seems like a thing we should fix.

Change-Id: Iec0382e2c8812f0713699ab5a0cd3180086ad433
diff --git a/tests/unit/test_cloner_cmd.py b/tests/unit/test_cloner_cmd.py
index 2d8747f..84bd243 100644
--- a/tests/unit/test_cloner_cmd.py
+++ b/tests/unit/test_cloner_cmd.py
@@ -26,7 +26,7 @@
 
     def test_default_cache_dir_empty(self):
         self.app.parse_arguments(['base', 'repo'])
-        self.assertEqual(None, self.app.args.cache_dir)
+        self.assertIsNone(self.app.args.cache_dir)
 
     def test_default_cache_dir_environ(self):
         try: