Add tox as development requirement

[psschwei]

Fixes #854

[Tansito]

Thanks @psschwei ! One question here, should we start pinning versions even for dev requirements, @IceKhan13 , @psschwei ?

[psschwei]

should we start pinning versions even for dev requirements

🤔 pinning makes things more reproducible / stable, but these are for building, so it might also be nice to be bleeding edge and/or pick up patches/fixes a little quicker ... I could go either way

[Tansito]

Well, in general we are relatively fast with updates with renovate. From time to time we got stuck in a version but I suppose it's something common. I don't have a strong opinion, either (feel free to merge it).

[psschwei]

Let's merge this and then pin them
(I did notice some weird stuff with using the most recent version of tox, which is why I had to do f66909b ... but I don't seem to be able to find the failed tests right now for some reason)