plugin: type-hint `priority` argument as `Literal[]`

[dgw]

Description

This will flag any use of unrecognized values for the callable priority. It makes a good intermediate step between the current free-form str accepted and probably turning it into an Enum later.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches
  • Indeed, if I change one of the priority decorators in core to an unlisted value, mypy raises an error 👍

[dgw]

@SnoopJ you can look at the revised docstring if/when you like. I'll go try to finish that Enum issue I started before our big conversation on IRC about other PRs started. 😁

[dgw]

OK, amended. You can do that final review you self-requested at your leisure, @Exirel 😃

[dgw]

Oh well. I guess this is a fight I can't win. I don't care anymore. Go with it.

If this is about the quotes you added a line note to, that was an error in my rebase. Should be fixed. 😅