[3.12] gh-100227: Lock Around Use of the Global "atexit" State (gh-105514) (gh-105517)

The risk of a race with this state is relatively low, but we play it safe anyway.
(cherry picked from commit 7799c8e678f759c7787785c6287140abe641d1b9)

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