Fixed running TransactionsPerRequestTests in isolation.

Caused by `contrib/auth/backends.py` changes in
cb7bbf97.

tests/handlers/tests.py

@@ -93,7 +93,7 @@ class HandlerTests(SimpleTestCase):
         self.assertEqual(response.status_code, 400)
 
 
-@override_settings(ROOT_URLCONF='handlers.urls')
+@override_settings(ROOT_URLCONF='handlers.urls', MIDDLEWARE=[])
 class TransactionsPerRequestTests(TransactionTestCase):
 
     available_apps = []