New:

• tox -e docker-slackware-current-standard
• tox -e docker-opensuse-15.4-standard

We remove centos-8, which can no longer be tested because the packages are no longer available (first noted in pypa/setuptools#2923 (comment))

Depends on #33196
Depends on #33103

CC: @dimpase @tobiasdiez

Component: porting

Author: Matthias Koeppe

Branch/Commit: e26283a

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/33306