aio-libs / multidict / 35846507942

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Drop the free-threaded Python 3.13 code path (#1524)

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## What do these changes do?

Support for free-threaded Python 3.13 was dropped in v6.8.0 (#1326),
once cibuildwheel stopped building cp313t wheels, but the C extension
kept a second free-threaded code path for interpreters without
`PyUnstable_TryIncRef()`, of which 3.13t is the only one. It was never
built in CI, never tested, and it duplicated `md_get_one()` and the
`md_contains()` dispatch outright.

`Py_GIL_DISABLED` now implies CPython 3.14 or newer. That assumption is
enforced by an `#error` in `freethreading.h`, so a free-threaded build
on 3.13t fails with a clear compiler message instead of silently
compiling an untested lock-free path (the vendored
`pythoncapi_compat.h` polyfills those calls, so it would otherwise
build). `_MD_HAVE_TRYINCREF` and every branch keyed on it are gone.

Only the C extension is affected. The pure-Python backend is
version-agnostic and unchanged, and builds with the GIL enabled still
go down to Python 3.10.

## Are there changes in behavior for the user?

No, on every supported build. The disassembly of `_multidict.so` is
identical before and after on both a free-threaded 3.14t build and a
GIL build. On an unsupported free-threaded 3.13t interpreter, building
from source now fails at compile time rather than producing an
untested extension.

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

No, it removes code.

## Related issue number

None; follow-up cleanup after #1326.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist — N/A, no behaviour change; the
      existing free-threading tests cover the surviving path
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` — N/A, no such file in this repo
- [x] Add a new news fragment into the `CHANGES/` folder
- [x] `make doc-spelling` passes and an... (continued)

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