Ensure no unexpected failures in treating deprecations as exceptions.

[mhvk]

The current del module.__warningregistry__ fails with a TypeError rather than AttributeError for h5py.
It seems reasonable to just skip trying to delete it on any exception. Tests run after this change. I don't really know how to add a test for this, though, short of testing with more Debian packages in our test grid.

Fixes #12388

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• 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.
• 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 a milestone set? Milestone must be set but astropy-bot check might be missing; 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]

Failures unrelated - will be fixed by #12390

[astrofrog]

Thanks for the fix! As an aside we really should consider deprecating our custom machinery for treating deprecation warnings as exceptions at some point since it is possible to do the same with pytest options...