ci grouping

[reaperhulk]

This uses workflow commands to do grouping, hiding the verbose tox output. The implementation is a bit inelegant here and it'd be better to try to get tox to support these commands directly long term.

Better ideas probably exist, but this at least serves as a PoC for making it much easier to read our CI output.

[alex]

As discussed on IRC, we're trending towards unreadable here and a pre-requisuite might be moving to something like nox.

[reaperhulk]

(Closing until there's ambition to do some nox work)