[3.12] gh-110395: invalidate open kqueues after fork (GH-110517) (#111745)

* [3.12] gh-110395: invalidate open kqueues after fork (GH-110517)

Invalidate open select.kqueue instances after fork as the fd will be invalid in the child.
(cherry picked from commit a6c1c04d4d2339f0094422974ae3f26f8c7c8565)

Co-authored-by: Davide Rizzo <sorcio@gmail.com>

* move assert to after the child dying

this is in `main` via https://github.com/python/cpython/pull/111816/files