* Fix bug where models with a class variable using a manager defined would interfere with other managers.
- Fill in the type argument for that particular instance of the manager, rather than modifying the bases of the Manager type.
- Instantiate a new Instance from determine_proper_manager_type so The code doesn't crash under mypy-mypyc.
* Use helpers.reparametrize_instance per review comment.
* Updated ignored errors in Django test for get_objects_or_404.
- For some reason, `Manager[nothing]` is now removed from expected types.
However, I think this makes sense anyway, as Manager is a subclass of QuerySet.
