Missing stubs (#467)

* adding missing db.backends typings and updated test

* added missing stubs

* reformatted files

* removed contrib.postgres.forms

* fixed test

django-stubs/contrib/gis/apps.pyi

@@ -0,0 +1,7 @@
+from django.apps import AppConfig as AppConfig
+from typing import Any
+
+class GISConfig(AppConfig):
+    name: str = ...
+    verbose_name: Any = ...
+    def ready(self) -> None: ...