fix reference to generic model manager

see also typeddjango/django-stubs#1684

src/base/tests/base.py

@@ -100,7 +100,7 @@ def assert_objects(self, model: Type[Model], count=1, filters=None) -> Model | Q
         if filters is None:
             filters = {}
 
-        objects = model.objects.filter(**filters)
+        objects = model._default_manager.filter(**filters)  # pylint: disable=protected-access
         self.assertEqual(len(objects), count)
         return objects[0] if count == 1 else objects