Modernized packaging needed

[dgw]

I can't get to all of the plugins at once, and we think this is a lesser-used one. But I intend to get to them all Eventually™ and creating an issue that I can self-assign helps with that.

This plugin also doesn't cap the version of sopel it requires to <8, so it won't refuse to work on the new Sopel 8 release or cause pip to silently downgrade an existing bot—which pushes it down the priority list, below other plugins that do do that.

(The project doesn't collect any analytics, so we have only anecdotal evidence and guesses to figure out which plugins are frequently used. Guessing is probably good enough. Probably.)

[dgw]

During modernization, the sopel.module import should be replaced by sopel.plugin. Deprecated use of bool with ValidatedAttribute should also be replaced with a BooleanAttribute configuration value:

sopel-rainbow/sopel_rainbow/__init__.py

Lines 30 to 31 in b96782b

	random_start = types.ValidatedAttribute('random_start', bool, default=False)
	"""Whether to randomize the start color."""

These updates will require bumping the minimum Sopel version to 7.1. Use of sopel.module is the reason for a note in the README & PyPI description that the plugin won't work with a future Sopel 9.0; module is scheduled for removal then.