Ignore PyPI anchors when checking links

[ncoghlan]

Closes #1744

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📚 Documentation preview 📚: https://python-packaging-user-guide--1767.org.readthedocs.build/en/1767/

[ncoghlan]

@jeanas @webknjaz This should be enough to unblock the rest of the pending PRs.

[ncoghlan]

Previously approved PRs that need another button click once this is merged:

• Add an initial specification doc for Dependency Groups #1623
• Use environment variables instead of contexts #1765

[webknjaz]

Thanks!