gh-159458: Fix free-threading data race in datetime strftime century normalization

[AnnaAr321]

In Modules/_datetimemodule.c, normalize_century() lazily initializes a function-local static int cache = -1; on the first strftime() call using %Y, %G, %F, or %C. Under free-threaded builds (Py_GIL_DISABLED), concurrent initial calls to strftime() race on reading and writing cache, triggering ThreadSanitizer data race warnings.
Use FT_ATOMIC_LOAD_INT_RELAXED and FT_ATOMIC_STORE_INT_RELAXED to atomically load and store cache in free-threaded builds with zero overhead in default GIL-enabled builds.

• Issue: Free-threading: data race on static cache in _datetime.normalize_century() #159458

[encukou]

Thank you.
AFAIK, this doesn't affect users in practice, so the NEWS blurb isn't really necessary. But since you already wrote it, we can keep it.

[miss-islington-app]

Thanks @AnnaAr321 for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Thanks @AnnaAr321 for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-159536 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-159537 is a backport of this pull request to the 3.14 branch.