TST: pin tox<4.0 to workaround incompatibility with tox-pyenv

[neutrinoceros]

This PR isn't meant to be merged. Rather, I intend to use it as a playground to discover bugs with the upcoming new major release of tox. Best case scenario nothing breaks and we can close this.

[neutrinoceros]

turns out we do have an issue here, in that tox-pyenv depends on the py library, but doesn't declare it explicitly as a dependency. It used to work because tox itself had py as a dependency but it looks like it's going to be dropped in tox 4.0

I will report this upstream to tox-pyenv, but the project looks very much unmaintained (no updates at all in 5 years), so I'm don't have much hope of getting an answer there.

edit: here's the report: tox-dev/tox-pyenv#21

[ngoldbaum]

I guess we could vendor it if it's unmaintained, I think the uses of py are pretty easy to patch out so we don't need to depend on that.

[neutrinoceros]

Actually I have hopes it might get answered as one of the maintainers also works very actively on tox itself. Hopefully we don't need to vendor this anything 🤞

[neutrinoceros]

tox 4.0 final was just released, I assume its too late to get the smooth transition I was hoping for
https://github.com/tox-dev/tox/releases/tag/4.0.0

[neutrinoceros]

yup. Now CI is plain broken... I opened a cross ref on the main repo tox-dev/tox#2611

[ngoldbaum]

I’d pin tox<4.0 for now

[neutrinoceros]

Going for it