aio-libs / multidict / 37833640970

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Fix the C extension build where char is unsigned (#1759)

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## What do these changes do?

The `v7.1.0` tag run failed to build the armv7l wheel, and the other
QEMU-emulated wheel jobs were cancelled with it, so nothing was
published. On ARM, PowerPC, s390x and RISC-V plain `char` is unsigned,
and initializing `empty_htkeys.indices` with `DKIX_EMPTY` (-1) trips
`-Wsign-conversion`, fatal under `-Werror` since #1611 turned
`-Wconversion` back on. Regular CI never builds those architectures, so
it surfaced only on the tag.

The fix casts the initializer entries to `char` explicitly. Runtime
behaviour was not affected: every read and write of the indices goes
through `int8_t*`, so the stored bytes are the same either way.

The changelog entry goes straight into the unpublished 7.1.0 section, as
RELEASE.md describes for a tag whose run never published anything; the
tag gets moved to the merge commit afterwards.

## Are there changes in behavior for the user?

No; the C extension builds again where `char` is unsigned.

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

No.

## Related issue number

Failed run:
https://github.com/aio-libs/multidict/actions/runs/37827427484

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist N/A, build fix; reproduced and
verified with `-funsigned-char` as described below
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt` N/A
- [ ] Add a new news fragment into the `CHANGES/` folder N/A, the entry
is added to the unreleased 7.1.0 section of `CHANGES.rst` directly
- [x] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_wordlist.txt`

Drafted with Claude Code (Claude Opus 5.5); reviewed by @asvetlov.

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Reproduced on x86_64 by building master with `CFLAGS=-f... (continued)

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