Fix ``treat_deprecations_as_exceptions`` to recognize Astropy deprecation warnings

Ironically, since ``AstropyDeprecationWarning`` does not inherit from ``DeprecationWarning`` (by choice, to avoid hiding the warnings), ``treat_deprecations_as_exceptions`` should be made to recognize this type of warning. Now 24 tests are failing, in:

* [ ] config
* [ ] coordinates
* [ ] io.fits
* [ ] modeling
* [ ] units
* [ ] wcs

I don't have time to go through these and fix them all now, so it would be great if people could go through these and open pull requests to my branch to fix some of them.

cc @embray @mdboom @eteq @mhvk