Update for Sopel 8 + `pyproject.toml` packaging style

[dgw]

When installing a package that still has setup.py and setup.cfg, like the projects this template generates, pip or setuptools (can't be bothered to track down which) complains that a setup.py installation is deprecated.

This template should be updated for Sopel 8 anyway, and that'll be a good time to switch it over to pyproject.toml, maybe with a Makefile to go along with it so the package can be built with make instead of python3 -m build.

[dgw]

Mostly done with #15, though it didn't add a Makefile. Leaving open until someone gets to that (@Exirel, are you interested?)

[Exirel]

Yeah, #15 does most of the necessary work I think. At least from the packaging side.

A Makefile template could be nice, after all I pretty much use the same on all my projects.

[Exirel]

Oh and maybe update the Readme of this repo:

Plugin template for Sopel 7 and higher. To use: