gh-149000: Fix race / leak in pickle XID per-interpreter cae

[CharlieKerfoot]

To fix issue #149000, I replace the load/store with an atomic compare-exchange. The fast path is a relaxed atomic load.
Population imports the attribute, then attempts to CAS it into the slot. On a lost CAS, the loser
Py_DECREFs its own reference and returns the winner's pointer (now owned by the slot).

Used a helper _cache_pickle_attr() because the logic was the same for the dumps and loads getters.

Tests

Existing test_interpreters / InterpreterPoolExecutor tests still pass under the GIL build
Same under --disable-gil

Fixed issue: #149000

• Issue: Race in pickle.dumps/loads per-interpreter cache leaks references #149000

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