remind: remove plugin reimplemented as external package w/better tests

[dgw]

Description

Tin. Development moved to https://github.com/sopel-irc/sopel-remind, with more comprehensive tests and support for running the plugin on Python 3.11+.(built-in has a regex pattern error).

Starting as a draft because of impending merge conflict from #2471 and waiting for a migration path for existing reminders to the new plugin (see sopel-irc/sopel-remind#1).

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
  • (It's very easy to license deletions 😼)
• [x] No issues are reported by make qa (runs make quality and make test)
• [x] I have tested the functionality of the things this change touches
  • By which I mean I've tested that remind is no longer included with the base Sopel installation, and the test suite was correctly removed (no leftovers causing failures post-deletion)

[Exirel]

Sopel Remind is now available as a package version 0.1.0!

[dgw]

Just need sopel-irc/sopel-remind#5 to be released and we're set to ship this. I take full responsibility for it not being merged yet, because I'm still making suggestions over there to this day. 😅

(I probably should've left remind alone in those recent mypy-cleanup branches to avoid this merge conflict, but it'll be fine.)

[dgw]

Migration feature is released upstream! Time to ready this.

[Exirel]

And it's done! sopel-remind 0.2.0 is up and allows bot's owner to migrate from the built-in plugin.