RFC: restrict utils.xml's extension modules to CPython's Limited API - #18249

Thank you for your contribution to Astropy! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the Astropy coding guidelines?
• Are tests added/updated as required? If so, do they follow the Astropy testing guidelines?
• Are docs added/updated as required? If so, do they follow the Astropy documentation guidelines?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions. Also see instructions for rebase and squash.
• Did the CI pass? If no, are the failures related? If you need to run daily and weekly cron jobs as part of the PR, please apply the "Extra CI" label. Codestyle issues can be fixed by the bot.
• Is a change log needed? If yes, did the change log check pass? If no, add the "no-changelog-entry-needed" label. If this is a manual backport, use the "skip-changelog-checks" label unless special changelog handling is necessary.
• Is this a big PR that makes a "What's new?" entry worthwhile and if so, is (1) a "what's new" entry included in this PR and (2) the "whatsnew-needed" label applied?
• At the time of adding the milestone, if the milestone set requires a backport to release branch(es), apply the appropriate "backport-X.Y.x" label(s) before merge.

👋 Thank you for your draft pull request! Do you know that you can use [ci skip] or [skip ci] in your commit messages to skip running continuous integration tests until you are ready?

Oh I just realized that PyTuple_SET_ITEM is monkeypatched on clang, which is hiding some remaining errors on my setup.

The compilation error on windows seems real. Will investigate.

Hum, I'll take a step back and try to get building on Windows + Limited API stable first in #18252

It might be good to add a comment about Py_VISIT but not critical so approving

I added a call to Py_VISIT following @da-woods' comment on a sibling PR: #18231 (comment)

Specifically, the source reference is https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_dealloc.

Heap types (Py_TPFLAGS_HEAPTYPE) must visit their type with:

Py_VISIT(Py_TYPE(self));

where IterParserType is a heap-type by virtue of being (with this refactor) defined dynamically with PyType_FromModuleAndSpec

I'll summarize this as an inline comment.

Thanks!