Add get_queryset to GenericAPIView (#26)

~~I took this from upstream and added the Mongo part, so hopefully it's alright~~, though my knowledge of the type system is a bit more limited than I'd like.

Nevermind, I went with a simpler approach as the approach from upstream wasn't passing tests for me, not exactly sure why.

rest_framework-stubs/generics.pyi

@@ -41,6 +41,7 @@ class GenericAPIView(views.APIView):
     @overload
     def paginate_queryset(self, queryset: MongoQuerySet[_T]) -> Optional[List[_T]]: ...
     def get_paginated_response(self, data: Any) -> Response: ...
+    def get_queryset(self) -> Union[QuerySet[_MT_co], MongoQuerySet[_T]]: ...
 
 class CreateAPIView(mixins.CreateModelMixin, GenericAPIView):
     def post(self, request: Request, *args: Any, **kwargs: Any) -> HttpResponse: ...