Integrate sphinx-issues; convert inline PyPI URLs to :pypi:

[sadiqkhzn]

Closes #1370.

Integrates sphinx-issues so the :pypi: role is available, per @webknjaz's feedback. The existing issue/pr/commit/user extlinks are moved over to sphinx-issues as well; gh stays in extlinks since sphinx-issues doesn't provide it.

Also converts ten inline pypi.org/project URLs in three files to use :pypi:, only in places where the visible link text already matched the project name.

One side effect to flag: :pr: usages in news.rst now render as (#N) instead of (PR #N). sphinx-issues 5.0.1 restricts issues_pr_prefix to #, @, or !, so the previous prefix can't be configured. Same end state as pypa/pip#12616. Happy to convert the 109 :pr: lines to inline links if you'd rather preserve the prose.

[sadiqkhzn]

Heads-up for reviewers: I noticed #2041 by @Herrtian also closes #1370 and adds the same one-line extlink alias. I missed it when picking this issue up — apologies. This PR additionally does the inline-link conversions (10 sites across 3 files), with rationale in the description for what was deliberately not converted.

I'm happy whichever path the maintainers prefer:

1. Merge Add PyPI extlink alias #2041 first and I'll rebase this to just the conversion sweep on top, or
2. Merge this and close Add PyPI extlink alias #2041 as superseded, or
3. Close this in favor of Add PyPI extlink alias #2041 and I'll open a separate PR for the conversions later.

Let me know which works best.

[webknjaz]

sphinx-issues does not register a pypi role, so it cannot fully cover this case. The pip project went the same route in pypa/pip#12616, keeping pypi as a plain extlink alongside sphinx-issues. This PR follows the same pattern.

That's not true. The plugin defines that role. Integrate it instead, as discussed in the issue.

Additionally, to keep this reviewable, could you delete most of the llm noise in the PR description?

[sadiqkhzn]

You're right, sorry — I was relying on outdated info from an earlier issue comment. sphinx-issues has had the :pypi: role since 4.0.0.

Pushed an update that integrates it: sphinx_issues is now in extensions, the pypi/issue/pr/commit/user extlinks are removed (only gh stays since sphinx-issues doesn't have it), and issues_user_uri is set to preserve the Sponsors URL. Trimmed the description too.

One thing to flag: sphinx-issues 5.0.1 (latest compatible with the pinned Sphinx 7.2.6) hardcodes issues_pr_prefix to one of #/@/!, so the 109 :pr: lines in news.rst now render as (#N) instead of (PR #N). Same outcome pip ended up with. If you'd rather keep the PR #N prose, I can convert those 109 lines to inline links in a follow-up commit — let me know.