Fix the doc

Don't use _PyObject_CAST() which doesn't exist in old Python versions.

Author: vstinner

Doc/whatsnew/3.11.rst

@@ -411,7 +411,7 @@ Porting to Python 3.11
       #if PY_VERSION_HEX < 0x030900A4 && !defined(Py_SET_TYPE)
       static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type)
       { ob->ob_type = type; }
-      #define Py_SET_TYPE(ob, type) _Py_SET_TYPE(_PyObject_CAST(ob), type)
+      #define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type)
       #endif
 
   (Contributed by Victor Stinner in :issue:`39573`.)