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/staticfiles/apps.pyi

@@ -1,15 +1,6 @@
-from typing import Any, Optional
+from typing import Any
 
 from django.apps import AppConfig
 
 class StaticFilesConfig(AppConfig):
-    apps: None
-    label: str
-    models: None
-    models_module: None
-    module: Any
-    path: str
-    name: str = ...
-    verbose_name: Any = ...
     ignore_patterns: Any = ...
-    def ready(self) -> None: ...