Is there any backport needed here? I'm not sure if the original PR made it in 3.13

No, this is a PEP 649 change and is only on main.

Heads-up: this appears to have caused some refleak buildbot failures, see for example https://buildbot.python.org/#/builders/1613/builds/116

Actually, on a second look, this part is technically unsafe:

else {
    Py_DECREF(dict);
    return PyDict_SetItem(dict, name, value);
}

If the dictionary's reference count is 1, then this will result in a use-after-free. Though, I guess that's never possible because the dictionary will be stored on the instance as well. In hindsight, there should have either been an assertion (or just not do it in the first place), but oh well.