update tox.ini, use tox testing in GitHub actions

[PetrDlouhy]

• The tox.ini file has been rewritten (updated, shortened, fixed)
• The TOX testing is used for GitHub tests. I think it will help to keep the tox.ini up to date and reduce code duplication between test.yaml and tox.ini.
• I added the 5.1 (pre-release) to the testing matrix to ensure the upcoming version will be supported
• flake8 testing is re-enabled and fixed. It could be compatible with black testing with correct settings, so I think we can have both.

All tests are passing on my repository: https://github.com/PetrDlouhy/django-multiselectfield/actions/runs/9300560098
There is no need to pull this before making a new release since there is no change in the working code.