Issue #26200: Added Py_SETREF and replaced Py_XSETREF with Py_SETREF

in places where Py_DECREF was used.

Include/object.h

@@ -851,18 +851,28 @@ PyAPI_FUNC(void) _Py_Dealloc(PyObject *);
  *
  * As in case of Py_CLEAR "the obvious" code can be deadly:
  *
- *     Py_XDECREF(op);
+ *     Py_DECREF(op);
  *     op = op2;
  *
  * The safe way is:
  *
- *      Py_XSETREF(op, op2);
+ *      Py_SETREF(op, op2);
  *
  * That arranges to set `op` to `op2` _before_ decref'ing, so that any code
  * triggered as a side-effect of `op` getting torn down no longer believes
  * `op` points to a valid object.
+ *
+ * Py_XSETREF is a variant of Py_SETREF that uses Py_XDECREF instead of
+ * Py_DECREF.
  */
 
+#define Py_SETREF(op, op2)                      \
+    do {                                        \
+        PyObject *_py_tmp = (PyObject *)(op);   \
+        (op) = (op2);                           \
+        Py_DECREF(_py_tmp);                     \
+    } while (0)
+
 #define Py_XSETREF(op, op2)                     \
     do {                                        \
         PyObject *_py_tmp = (PyObject *)(op);   \