Type conflict between base tools.jobs.Scheduler class and plugins.jobs.Scheduler subclass

[dgw]

Raised in #2471. Band-aided with a type: ignore[assignment] comment, but we should really redesign this part of the API.

@Exirel pointed out that Sopel doesn't really need a non-plugin job scheduler any more, since it can just use the asyncio event loop now. Since tools.jobs is already documented as internal tools that plugin developers shouldn't use, any reason we shouldn't just drop the tools.jobs module and move the still-useful parts into plugins.jobs?

(tools.jobs says "As of Sopel 5.3, this is an internal tool […]", so that's several years of warning that plugin authors shouldn't use it. The Scheduler class also used to be a JobScheduler before some refactoring years ago, so code that tries to use the even older version that wasn't explicitly documented as being internal will fail.)