aio-libs / yarl / 36879979364

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Narrow the backslash category of the conformance report (#1959)

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## What do these changes do?

The `backslash` category of the conformance report caught every input
containing a backslash. Because it is checked early, most of its rows
were really about something else: Windows drive letters in `file:` URLs,
quoting of punctuation in paths, or backslashes in non-special URLs
where WHATWG treats them as ordinary characters.

The category is now `backslash read as a slash` and only matches when
the effective scheme is special, a backslash appears before the query or
fragment, and the input is within the RFC 3986+3987 grammar once those
backslashes become slashes. It is checked after the host and `file
scheme` categories. The other rows move to `file scheme` or `characters
outside the RFC 3986+3987 grammar`.

## Are there changes in behavior for the user?

No. Only the category column of `tools/conformance/REPORT.md` changes;
no case moves between sections.

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

No, it is one predicate in the report generator.

## Related issue number

N/A

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist: N/A, report tooling only;
`compare.py --check` covers it
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`: N/A
- [x] Add a new news fragment into the `CHANGES/` folder

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

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

Report: `PYTHONPATH=$PWD python tools/conformance/compare.py --check`
passes; a diff of REPORT.md with the category column stripped shows no
row changes.
Spelling: `make doc-spelling` passes.
Lint: pre-commit passes on the commit.
</details>

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