Use tox release candidates in pytest CI

[obestwalter]

With tox 2.8 we start using release candidates and it would be nice if you could start using them in pytest CI tests.

All that needs to be done is to add the --pre flag to the install command in the CI configs. I can open a PR if there are no objections.

[RonnyPfannschmidt]

main question is what will that cost pytest - while i trust in the stability of tox - i don't see the direct need to make pytest a "testbed"

on the other hand its a quite nice fallout connection

[obestwalter]

As we have some developer overlap I thought it might not hurt to propose to use tox release candidates here. The cost is that maybe sometimes CI builds fail due to a new bug in a tox rc, which is hopefully easy to recognize then.

[RonnyPfannschmidt]

yup - i think i'm +0 on this one, ^^

[nicoddemus]

I'm 👍!

[obestwalter]

Hey thanks :)