deprecate comparablex509

[bmw]

this is an alternate to #206

one thing i noticed is that all code deprecated in #206 uses ComparableX509 in some way so maybe we could just put this warning in one spot. i also expanded the warning text a bit and put something about it in the changelog

initially i also wanted to make use of code like https://github.com/certbot/josepy/blob/main/src/josepy/util.py#L259-L293 so users who have code like from josepy.util import ComparableX509 get a warning about that problematic import as well, but it's kind of annoying because we also reexport ComparableX509 in __init__.py. we could also do the same thing there, but it gets a little messy because with a naive approach __init__.py would be generating its own warning for importing josepy.util.ComparableX509 on top of generating another warning if anyone touches the reexported attribute. i personally think all that is probably too much to be worth the effort and this warning in ComparableX509.__init__ is good enough

erica, feel free to suggest changes or update one of your own PRs with proposed alternatives

[ohemorange]

I like this approach better! I agree it's not worth it to warn on import, this seems fine to me. I did just remember though that sphinx has a deprecated tag, which we should add.