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Stop case-folding istr keys in case-sensitive MultiDict (#1457)

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## What do these changes do?

`_key_to_identity` in `multidict/_multilib/hashtable.h` returned the
cached lower-cased `canonical` for an `istr` key, so a case-sensitive
`MultiDict` matched that key case-insensitively. The pure Python
`_CSMixin._identity` (`multidict/_multidict_py.py:596`) has no `istr`
branch and returns the key unchanged, so the two backends disagreed.

The C container also contradicted itself:

```pycon
>>> d = MultiDict([(istr("A"), 1)])
>>> list(d.keys())
['A']
>>> "A" in d
False
>>> "a" in d
True
```

Dropping the branch lets `istr` fall through to the existing
`PyUnicode_Check` path at `hashtable.h:141`, which preserves case
exactly as the other two `str` branches in that same function already
do. The case-insensitive sibling `_ci_key_to_identity` is untouched.

## Are there changes in behavior for the user?

Yes. A case-sensitive `MultiDict` no longer folds case for `istr` keys.
That restores the documented contract in `docs/multidict.rst`, added by
#1403 when #1397 was closed: "case-insensitive matching is handled by
:class:`CIMultiDict`, not by ``istr`` equality". `CIMultiDict` is
unaffected.

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

No; it removes three lines and adds two tests to the existing
parametrized suite.

## Related issue number

Related to #1397 and #1403, which state the contract this restores. No
issue to close.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [x] Documentation reflects the changes (`docs/multidict.rst` already
states the restored contract; no change needed)
- [ ] 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
- [ ] `make doc-spelling` passes and any new technical words are added
to `docs/spelling_word... (continued)

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