votable: Add support for VOTable 1.6 UTF-8 char encoding - #18987

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?

Thank you @stvoutsin, I totally missed this!

I'm adding xref to ivoa-std/VOTable#71; and also adding the keep-open label, as my understanding is that we use this PR as a reference implementation but don't merge it in until the upstream decision has been made. Or maybe even until it goes through the TCG.

Hi humans 👋 - this pull request hasn't had any new commits for approximately 5 months. I plan to close this in a month if the pull request doesn't have any new commits by then.

In lieu of a stalled pull request, please consider closing this and open an issue instead if a reminder is needed to revisit in the future.

If this PR still needs to be reviewed, as an author, you can rebase it to reset the clock.

If you believe I commented on this pull request incorrectly, please report this here.

ping @stvoutsin - could you rebase so the stale bot goes away. And anyways, we're close to feature freeze and I believe the consensus was to try to get all the votable 1.6 relevant stuff in this release, if possible.

I've spotted a few issues after looking at the latest version of the VOTable 1.6 text. I will update accordingly