Extend linkcheck GET fallback to handle TooManyRedirects (swev-id: sphinx-doc__sphinx-8475)

[casey-brooks]

Summary

• extend the HEAD→GET fallback to also catch requests.exceptions.TooManyRedirects
• add regression coverage for successful fallback and persistent failures after GET
• introduce a focused linkcheck test root for TooManyRedirects scenarios

Reproduction

1. Create /workspace/repro with the minimal conf.py and index.rst described in the issue.
2. Run sphinx-build -b linkcheck /workspace/repro /workspace/repro/_build/linkcheck -v.

Observed failure (before fix):

index.rst:4: [broken] https://idr.openmicroscopy.org/webclient/?show=well-119093: Exceeded 30 redirects.

Python traceback showing TooManyRedirects on HEAD:

Traceback (most recent call last):
  File "/workspace/repro/too_many_redirects.py", line 7, in <module>
    requests.head(URL, allow_redirects=True, timeout=30)
  File "/workspace/sphinx/.venv/lib/python3.11/site-packages/requests/api.py", line 100, in head
    return request("head", url, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/sphinx/.venv/lib/python3.11/site-packages/requests/api.py", line 59, in request
    return session.request(method=method, url=url, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/sphinx/.venv/lib/python3.11/site-packages/requests/sessions.py", line 589, in request
    resp = self.send(prep, **send_kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/sphinx/.venv/lib/python3.11/site-packages/requests/sessions.py", line 724, in send
    history = [resp for resp in gen]
              ^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/sphinx/.venv/lib/python3.11/site-packages/requests/sessions.py", line 724, in <listcomp>
    history = [resp for resp in gen]
              ^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/sphinx/.venv/lib/python3.11/site-packages/requests/sessions.py", line 191, in resolve_redirects
    raise TooManyRedirects(
requests.exceptions.TooManyRedirects: Exceeded 30 redirects.

Validation

• pytest tests/test_build_linkcheck.py -q
• pytest -q (fails on existing branch due to longstanding mismatches in autodoc, intl, and html code-block expectations — unchanged by this patch)
• sphinx-build -b linkcheck /workspace/repro /workspace/repro/_build/linkcheck -v

Post-fix output excerpt:

(line    4) ok        https://idr.openmicroscopy.org/webclient/?show=well-119093

[casey-brooks]

Test & Lint Summary:\n- \ → 19 passed, 0 failed\n- \ → fails on existing baseline (html code-block lineno assertions, autodoc expectations, intl translation checks); unchanged by this patch\n- No standalone lint command provided in this branch

[noa-lucent]

Extends the HEAD→GET fallback to cover TooManyRedirects and adds solid regression tests. Verified the new fixtures and monkeypatch-based tests; everything looks clean.