Fix type errors on other models' managers when using objects = models.Manager() in Model. (#34)

* 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.

scripts/typecheck_tests.py

@@ -199,9 +199,9 @@ IGNORED_ERRORS = {
     ],
     'get_object_or_404': [
         'Argument 1 to "get_object_or_404" has incompatible type "str"; '
-        + 'expected "Union[Type[<nothing>], Manager[<nothing>], QuerySet[<nothing>]]"',
+        + 'expected "Union[Type[<nothing>], QuerySet[<nothing>]]"',
         'Argument 1 to "get_list_or_404" has incompatible type "List[Type[Article]]"; '
-        + 'expected "Union[Type[<nothing>], Manager[<nothing>], QuerySet[<nothing>]]"',
+        + 'expected "Union[Type[<nothing>], QuerySet[<nothing>]]"',
         'CustomClass'
     ],
     'get_or_create': [