meta: modernize the project and its packaging

[Exirel]

This is a long overdue PR to modernize this plugin:

• the instructions to run tests didn't work with latest version of pytest
• pylint config was severely outdated
• coverage wasn't 100%, unacceptable (no actually it was a choice with a plan to fix it quickly)
• no more setup.py, welcome to modern Python packaging with pyproject.toml
• and now, with an actual CI that runs for every PR (including this one, in theory)
• a slightly reworked Makefile for good measure
• within a virtualenv, you can now do make develop qa build then make release if you feel like it
• using a .dev version for now

[Exirel]

Note: Python 3.7 to 3.10 only, as Py 3.6 wasn't supported anymore, and Py 3.11 isn't supported by Sopel 7.1.