Using assertIsNone(xxx) instead of assertEqual(None, xxx)

Following OpenStack Style Guidelines:
[1] https://docs.openstack.org/developer/hacking/#unit-tests-and-assertraises
[H203] Unit test assertions tend to give better messages for more specific
assertions. As a result, assertIsNone(xxx) is preferred over
assertEqual(None, xxx) and assertIs(xxx, None)

Change-Id: Id0e54ddeb461e56fbc8ec3a4bbc8a581f11430df

oslo_db/tests/sqlalchemy/test_models.py

@@ -211,7 +211,7 @@ class SoftDeleteMixinTest(test_base.DbTestCase):
         self.session.add(m)
         self.session.commit()
         self.assertEqual(0, m.deleted)
-        self.assertIs(None, m.deleted_at)
+        self.assertIsNone(m.deleted_at)
 
         m.soft_delete(self.session)
         self.assertEqual(123456, m.deleted)