tox.ini, GH Actions: Add ubuntu-impish, linuxmint-20.2, fedora-35 #31831
Description
https://communityblog.fedoraproject.org/fedora-linux-35-development-schedule/
Component: porting
Author: Matthias Koeppe
Branch/Commit: b3a3c6d
Reviewer: Jonathan Kliem
Issue created by migration from https://trac.sagemath.org/ticket/31831