Bug report

Bug description:

While working on #126908, @vstinner noted that this part of PyUnstable_AtExit looks wrong:

This will result in loss of callbacks after one has been stored, because the second-to-last one is always overwritten. Noted in gh-118915, PyUnstable_AtExit is also undocumented. I'll work on fixing both.

CPython versions tested on:

3.13, 3.14, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-127791: Fix, document, and test PyUnstable_AtExit #127793
• [3.13] gh-127791: Fix, document, and test PyUnstable_AtExit (GH-127793) #127819