[3.14] gh-148393: Use atomic ops on _ma_watcher_tag in free threading build (gh-148397) (#148451)

Fixes data races between dict mutation and watch/unwatch on the same dict.
(cherry picked from commit 3ab94d684286b49144bf2e43cc1041f3e4c0cda8)

Co-authored-by: Sam Gross <colesbury@gmail.com>