Fix support of custom WCS mapping.

[cmarmo]

Description

This pull request is to address issue #15625.
After the introduction of Solar System frames the creation of custom WCS frame with ctype yzLT, yzLN was no longer possible.
This pr reverts the previous behaviour.

Fixes #15625

• By checking this box, the PR author has requested that maintainers do NOT use the "Squash and Merge" button. Maintainers should respect this when possible; however, the final decision is at the discretion of the maintainer that merges the PR.

[github-actions]

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?
• Is a milestone set? Milestone must be set but we cannot check for it on Actions; do not let the green checkmark fool you.
• 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.

[pllim]

Thanks! Is this a bug or a new feature?

[cmarmo]

This is a fix for a regression introduced in #14820 ... I guess it would be nice to have it in 6.0 ... but then I don´t know how the Changelog should be managed...

[saimn]

@cmarmo - Please add a changelog entry with the usual way, it will then be backported with the PR.

[cmarmo]

... I have no clue about the failures...

[neutrinoceros]

@cmarmo I think failures are unrelated to your PR, just bad luck on timing. See pypa/setuptools#4126

[neutrinoceros]

Turns out it's actually a simple fix away on our side: #15634

[svank]

Can confirm this fixes #15625 for me. Thanks for the quick response, @cmarmo !

[cmarmo]

Thanks @neutrinoceros for pointing out the issue and for the follow-up leading to the fix!
I've triggered the CI, seems like everything is back to normal after pypa/setuptools#4127.

[saimn]

@nden @mcara - If you have time to review, would be nice to get this in 6.0.

[pllim]

I am going to approve by proxy based on positive feedback from @svank but I will let others decide when to merge. Thanks!

[mcara]

LGTM