Tests and validation improvements

Before merging !6 (merged) we need to improve tests and perhaps have a documentation to explain how to run all tests before a release.

In particular, all tests shall be ran for all supported versions

About linting, this project started by using flake8. It now also uses ruff. I agree to drop support of flake8 and move to ruff. Ruff complies to black's default. One notable thing is that lines length limit is 88 while this project was limiting to 80.

Things to do:

• move flake8 to ruff
• complies code to ruff (including 88 columns lines)
• change tox to run ruff instead of flake8
• explain how to run tox for all versions (be generic, no distro-based solution) (pyenv probably)