Add test to import all modules to check validity of stubs (#56)

* add import_all.test builder

* fix errors

* fix typechecking errors

* fix migrations typechecking

django-stubs/contrib/messages/storage/fallback.pyi

@@ -1,11 +1,8 @@
-from typing import Any, Optional
+from typing import Any
 
 from django.contrib.messages.storage.base import BaseStorage
 
 class FallbackStorage(BaseStorage):
-    added_new: bool
-    request: WSGIRequest
-    used: bool
     storage_classes: Any = ...
     storages: Any = ...
     def __init__(self, *args: Any, **kwargs: Any) -> None: ...