Correct parsing of CDS units in the presence of multiple divisions

[mhvk]

Description

This pull request is to address the fact that units with multiple divisions, like "km/s/Mpc", are currently incorrectly parsed in the CDS format (as equivalent to "km Mpc/s" instead of "km/(s.Mpc)").

Fixes #14355

[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 "When to rebase and squash commits".
• 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.

[mhvk]

Note that I thought this change was worthwhile specifically mentioning under io.ascii, since it affects that package most. I have not included a specific test, since I couldn't easily find an existing table that had multiple solidi.

[pllim]

What about that table given in #14355 ?

[mhvk]

I'd prefer to use an actually published table... Note that I used the unit that causes the problems in the tests, so the table from #14355 is now fine.

[pllim]

I don't grok the parsing internals but I trust the tests. I am not sure if you are waiting for original bug report author to review or not, so I am approving but will let you merge when you are ready. Thanks for the quick fix!

[mhvk]

@jensmelinder - did you encounter an existing table in which the units had double slashes? If so, I'd be happy to add its readme to the tests. If not, I will merge this. Thanks!