[3.13] gh-120838: Add _PyThreadState_WHENCE_FINI (gh-121013)

We also add _PyThreadState_NewBound() and drop _PyThreadState_SetWhence().

This change only affects internal API.

(cherry picked from commit a905721b9c5c15279e67c2f7785034b7356b2d46, AKA gh-121010)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>