)

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## What do these changes do?

Follow-up to #13855 for aio-libs/yarl#1937. With that change, yarl's
default WHATWG mode reads a host from `http:host/p`, `http:/host/p` and
`http:///host/p`, as browsers do. The Python and Cython request parsers
passed an absolute-form request target to `URL()` and rejected it only
if yarl found no host. They now first check that the target has `//`
followed by a non-empty authority, so `GET http:///protected` and `GET
http:/protected` stay rejected as RFC 9110 requires, whatever yarl
version is installed.

The client tests used `http:///example.com` and `http:///path` as URLs
without a host. They now use `http://` where an invalid URL is needed,
and `http:///example.com` is dropped from the invalid URL test data.
With yarl#1937 it is `http://example.com/`, and as a redirect it leads
to `example.com`, as in browsers.

## Are there changes in behavior for the user?

No with the released yarl, where such targets were already rejected.
With yarl#1937, the server keeps rejecting absolute-form targets without
an authority instead of reading a host from them.

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

No, it is one string check shared by both parsers.

## Related issue number

Follow-up to #13855; needed by aio-libs/yarl#1937.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist
- [ ] Documentation reflects the changes: N/A, no documented behavior
changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`: N/A, already listed
- [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>

Tests with the Cython extensions built (llhttp generated, `make
cythonize`, `build_ext --inplace`): `pytest tests -n auto`, 5246 passed,
22 skipped, 17 xfailed, both with yarl master (f761c37) and with the
yarl#1937 branch (8d0f111). Pure-Python run of `test_http_parser.py`,
`test_client_functional.py` and `test_client_session.py` with the
yarl#1937 branch: 884 passed.
Lint: black, isort, codespell passed. The local pre-commit flake8 hook
fails to load `flake8-requirements` (`pkg_resources` missing), so flake8
(E, F, W) was run directly and passed. yesqa was skipped for the same
reason, because it wrongly strips `# noqa: I900`.
</details>

(cherry picked from commit ce960dd)