Make APIView.content_negotiation_class type, not str (#155)

Author: flaeppe

rest_framework-stubs/views.pyi

@@ -37,7 +37,7 @@ class GenericView(Protocol):
 class APIView(View):
     args: Any = ...
     authentication_classes: Sequence[Type[BaseAuthentication]] = ...
-    content_negotiation_class: Optional[str] = ...
+    content_negotiation_class: Type[BaseContentNegotiation] = ...
     format_kwarg: Any = ...
     headers: Dict[str, str] = ...
     kwargs: Any = ...