PR Summary

This makes our Artists very picky about being re-added to Axes / Figures once they have been removed. Currently there is enough checking in place that it should not be possible to try and add the Artist to more than one simultaneously, but it is not clear we actually do enough clean up to make these re-added Aritsts actually work. I suspect in most cases where users are removing / readding Aritsts they would be better off usingset_visible(), but I have no idea how common it actually is.

Open questions:

• should this actually start a deprecation cycle where we warn about "going through None" on Artists?
• should this wait for theArtistList deprecation to expire (if we are ever going to expire those)?

I think this is sufficiently tested, but needs API change notes, but waiting for the above decisions before writing them.

In any case, the change to the OffsetBox test should probably be pulled out and if we do not take this, find a way to make that error out for maybe other reasons.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (andpytest passes)
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).

Release Notes

• API changes are marked with a.. versionchanged:: directive in the docstring and documented indoc/api/next_api_changes/