Deprecate yapsy compatibility function names.

[aknrdureegaesr]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable). Not applicable.
• I tested my changes.

Description

Output a deprecation warning for people that use the old, Yapsy function names, so those can eventually be removed.

It is ok with me if my work is in the end squashed into one commit, together with the other work on the new-plugin-mananger branch.

[Kwpolska]

Sorry, but the proposed implementation is far too complicated. Only two plugins use this now, I’ll implement something to show a warning unconditionally instead of showing it only once and with traceback magic.

[aknrdureegaesr]

Fine with me.

Only two plugins use this now,

Epistemological remark: I don't think you can know all Nikola plugins 🤓.

[Kwpolska]

Epistemological remark: I don't think you can know all Nikola plugins 🤓.

Well, two plugins in the getnikola/plugins repository :)