Improve GenericAPIView.serializer_class classvar type generics (#672)

The type for the attribute does not match what the `get_serializer_class` function returns.

Thus, modifying it to match so it is `type[BaseSerializer[_MT_co]]`

Unfortunately this doesn't solve the issue of projects programmatically returning different serializers in `get_serializer_class` such that one option would be: `return self.serializer_class` such that it would be technically incompatible with the typing. But that seems outside the scope of this change and there are other ways to accomplish it (e.g. holding known serializer classes in a `dict` on the class as an attribute).

Author: jheld

rest_framework-stubs/generics.pyi

@@ -33,7 +33,7 @@ class BaseFilterProtocol(Protocol[_MT_inv]):
 
 class GenericAPIView(views.APIView, UsesQuerySet[_MT_co]):
     queryset: QuerySet[_MT_co] | Manager[_MT_co] | None
-    serializer_class: type[BaseSerializer] | None
+    serializer_class: type[BaseSerializer[_MT_co]] | None
     lookup_field: str
     lookup_url_kwarg: str | None
     filter_backends: Sequence[type[BaseFilterBackend | BaseFilterProtocol[_MT_co]]]