Add return type for get_metadata_routing()

Author: tmke8

stubs/sklearn/utils/_metadata_requests.pyi

@@ -71,7 +71,7 @@ class RequestMethod:
 
 class _MetadataRequester:
     def __init_subclass__(cls, **kwargs): ...
-    def get_metadata_routing(self): ...
+    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`
     # methods are dynamically generated, and they complain that they are