[Python-Dev] Add Py_SETREF and Py_XSETREF to the stable C API

Macros Py_SETREF and Py_XSETREF were introduced in 3.6 and backported to all maintained versions ([1] and [2]). Despite their names they are private. I think that they are enough stable now and would be helpful in third-party code. Are there any objections against adding them to the stable C API? [3]

[1] https://bugs.python.org/issue20440 [2] https://bugs.python.org/issue26200 [3] https://bugs.python.org/issue31983