Only accept type[Self] in the object.__class__ setter (#13021)

stdlib/builtins.pyi

@@ -107,7 +107,7 @@ class object:
     @property
     def __class__(self) -> type[Self]: ...
     @__class__.setter
-    def __class__(self, type: type[object], /) -> None: ...
+    def __class__(self, type: type[Self], /) -> None: ...
     def __init__(self) -> None: ...
     def __new__(cls) -> Self: ...
     # N.B. `object.__setattr__` and `object.__delattr__` are heavily special-cased by type checkers.