linkcheck: treat URI schemes as case-insensitive (RFC 3986)

[sudorm-rf0]

Hi,

This fixes a linkcheck bug where URIs with an uppercase scheme (e.g. FTP://example.test/file) were reported as [broken], while the lowercase form ftp:// was correctly reported as [unchecked].

AI assistance disclosure (per the AI policy): I used an AI coding assistant to help produce the initial patch and the regression test. I have reviewed the diff, understand exactly what it does, and can explain it below. I am opening this PR myself after checking it.

What's going on / why I think this is the right fix

Per RFC 3986 §3.1, URI schemes are case-insensitive, so FTP:// and ftp:// are the same scheme. Sphinx's linkcheck decides whether a link is "external" (a non-HTTP scheme) with this regex in sphinx/builders/linkcheck.py:

uri_re = re.compile('([a-z]+:)?//')

Because the character class is [a-z] only, an uppercase scheme like FTP:// never matches, so linkcheck does not recognise it as an external link. It then falls through to the local-file existence check and reports [broken] for a URI that is actually a remote FTP link.

The fix is a one-liner: make the regex case-insensitive.

uri_re = re.compile('([a-z]+:)?//', re.IGNORECASE)

Now FTP://, SSH://, etc. are treated exactly like their lowercase forms and reported as [unchecked]. http:/https: links are unaffected (they are still checked normally), and nothing else in the matching logic changes.

How I verified it

• Added a regression test tests/test_builders/test_build_linkcheck.py::test_linkcheck_uppercase_scheme_is_unchecked covering FTP://, ftp://, and SSH://. It reports broken on the unpatched code and unchecked with the fix.
• pytest tests/test_builders/test_build_linkcheck.py → 51 passed, no regressions.
• ruff check / ruff format --check clean on the changed files.

Fixes #14541.