Keep old reference to fix PEP 621 build

[hugovk]

The removal of the .. _declaring-project-metadata:reference in #1396 has broken the PEPs build because PEP 621 used to link here via Intersphinx:

peps/pep-0621.rst:23: WARNING: undefined label: 'packaging:declaring-project-metadata'

https://github.com/python/peps/actions/runs/7092751804/job/19304712978

https://github.com/python/peps/blob/ff0702ebfce6e5196a0969e609d2a45e5bd41843/peps/pep-0621.rst?plain=1#L23

I think retaining the old reference should fix it.

Instead/also, we could update PEP 621 to point to the new reference. I don't really mind, but there could be other sites also linking via intersphinx so perhaps this is the more general fix.

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📚 Documentation preview 📚: https://python-packaging-user-guide--1434.org.readthedocs.build/en/1434/

[webknjaz]

@hugovk looks like maybe an empty line is needed?

[hugovk]

No, they can be next to each other, see for example https://github.com/python/devguide/blob/main/documentation/start-documenting.rst?plain=1

The other warnings were also introduced by the re-org in #1396 plus another PR landing at the same time, see #1396 (comment).

Edit: And will be fixed by #1433.

[jeanas]

I'm not against preserving labels when doing reorganizations. I want to point out, though, that if we want to treat our Intersphinx "interface" as public API, we also need to avoid removing any rst files, using orphan pages instead, since they can also be referenced with :doc:. This relates to the discussion in #1401.

[hugovk]

We've fixed this for the PEPs by updating the reference:

python/peps#3563

However, we're still running into build failures when updating PRs created more than a week ago, for example:

• PEP 726: Fix error message description python/peps#3562
• CanonicalPyPASpecBanner or Intersphinx fails on RTD? python/peps#3583

We can fix them by updating them from main, however it would still be useful to merge this to preserve the old name, and avoid surprises for others.

Thank you!

[willingc]

Thanks @hugovk.