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.

mypy_django_plugin/main.py

@@ -64,13 +64,20 @@ def determine_proper_manager_type(ctx: FunctionContext) -> Type:
     if not isinstance(ret, Instance):
         return ret
 
+    has_manager_base = False
     for i, base in enumerate(ret.type.bases):
         if base.type.fullname() in {helpers.MANAGER_CLASS_FULLNAME,
                                     helpers.RELATED_MANAGER_CLASS_FULLNAME,
                                     helpers.BASE_MANAGER_CLASS_FULLNAME}:
-            ret.type.bases[i] = Instance(base.type, [Instance(outer_model_info, [])])
-            return ret
-    return ret
+            has_manager_base = True
+            break
+
+    if has_manager_base:
+        # Fill in the manager's type argument from the outer model
+        new_type_args = [Instance(outer_model_info, [])]
+        return helpers.reparametrize_instance(ret, new_type_args)
+    else:
+        return ret
 
 
 def return_user_model_hook(ctx: FunctionContext) -> Type: