Migrate to tox-dev and v4 support

[gaborbernat]

Hello, would you consider moving the project under the tox-dev umbrella? See documentation under https://tox.readthedocs.io/en/rewrite/plugins.html#adoption-of-a-plugin-under-tox-dev-github-organization

Furthermore, tox v4 is getting ready and we'd like to make sure this plugin is supported from day 1, we're collecting feature gaps for this under tox-dev/tox#1974. Would be great if you could join our development chat under https://discord.gg/tox so we can assist with this. If you do so please drop in a line in the #plugin chat with the name of the repository you maintain. Thanks!