[pickle] Fix _Pickle.reducer_override annotation (#13545)

stdlib/_pickle.pyi

@@ -61,7 +61,6 @@ class PicklerMemoProxy:
 class Pickler:
     fast: bool
     dispatch_table: Mapping[type, Callable[[Any], _ReducedType]]
-    reducer_override: Callable[[Any], Any]
     bin: bool  # undocumented
     def __init__(
         self,
@@ -79,6 +78,10 @@ class Pickler:
 
     # this method has no default implementation for Python < 3.13
     def persistent_id(self, obj: Any, /) -> Any: ...
+    # The following method is not defined on _Pickler, but can be defined on
+    # sub-classes. Should return `NotImplemented` if pickling the supplied
+    # object is not supported and returns the same types as `__reduce__()`.
+    def reducer_override(self, obj: object, /) -> _ReducedType: ...
 
 @type_check_only
 class UnpicklerMemoProxy: