aio-libs / multidict / 35761211686

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Fix a data race on the MultiDict version counter (#1514)

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## What do these changes do?

`md_version()` read `md->version` with a plain, non-atomic load, while
every mutation site wrote it with a plain store under the object's own
critical section. On a free-threaded build, `getversion()` (which calls
`md_version()` without taking that critical section) could race a
concurrent writer. Both sides now use the relaxed atomic load/store pair
already used for the object's item count, so the read and every write
site go through `atomic_load_uint64_relaxed()` /
`atomic_store_uint64_relaxed()`.

## Are there changes in behavior for the user?

No. `getversion()` still returns the same opaque, per-mutation token;
this only removes a data race on free-threaded builds.

## Is it a substantial burden for the maintainers to support this?

No, it mirrors the existing atomic-op pattern in `atomic_helpers.h`.

## Related issue number

Related to #1444, which fixed the analogous race on the shared global
counter but left the per-object `md->version` field itself unaddressed.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

Drafted with Claude Code (Sonnet 5); reviewed by asvetlov.

<details>
<summary>Agent run details (optional, for reviewers)</summary>

Tests: `pytest -q`, all green on the C extension under a normal GIL
build (2160 passed), a free-threaded 3.14t build (2148 passed), and the
pure-Python build (2160 passed, unchanged since this is C-extension
only).
New test:
`tests/test_free_threading.py::test_race_condition_getversion_vs_mutation`,
marked `c_extension`.
Sanitizer: confirmed with `MULTIDIC... (continued)

765 of 1530 branches covered (50.0%)

Branch coverage included in aggregate %.

41 of 41 new or added lines in 1 file covered. (100.0%)

6769 of 7053 relevant lines covered (95.97%)

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