Feature or enhancement

Now that #108337 is done, we have three atomic headers. We should be able to remove pycore_atomic.h and pycore_atomic_funcs.h and replace their usages with calls to pyatomic.h.

cc @vstinner

Linked PRs

• gh-109693: Remove pycore_atomic_funcs.h #109694
• gh-109693: Use pyatomic.h for signal module #110477
• gh-109693: Use pyatomic.h for signal module #110480
• gh-109693: Update pycore_interp.h to use pyatomic.h #110604
• gh-109693: Update _gil_runtime_state.last_holder to use pyatomic.h #110605
• gh-109693: Update _gil_runtime_state.locked to use pyatomic.h #110836
• gh-109693: Update pyruntimestate._finalizing to use pyatomic.h #110837
• gh-109693: Remove pycore_atomic.h #110992