Add explicit None return annotation on _MetadataRequester.__init_subclass__

Author: debonte

stubs/sklearn/utils/_metadata_requests.pyi

@@ -70,7 +70,7 @@ class RequestMethod:
     def __get__(self, instance, owner): ...
 
 class _MetadataRequester:
-    def __init_subclass__(cls, **kwargs): ...
+    def __init_subclass__(cls, **kwargs) -> None: ...
     def get_metadata_routing(self) -> MetadataRequest: ...
     # This code is never run in runtime, but it's here for type checking.
     # Type checkers fail to understand that the `set_{method}_request`