Rev up base supported versions of OS and Python - including in CI

[pombredanne]

See also aboutcode-org/skeleton#46

We could still support some Ubuntu 16 by using a container image as opposed to CI-provided images... but these would be harder to keep up to date since there are no security updates provided by default. So I am in favor of:

• move the base tests to Ubuntu 18 LTS on all supported Python versions
• add or keep Debian buster, Ubuntu 20 LTS, Fedora FC34 to the smoke tests stack on one Python version (their default)
• consider adding nix, CentOS, Arch and Suse to the mix in the smoke tests stack on one Python version
• drop Python 3.6 everywhere
• move to Python 3.8 Move to Python 3.8 as a the base Python version for release and testing #2664
• add Python 3.10.0 Add support for the newly released Python 3.10 and 3.11 #2730

[pombredanne]

We used to have a way to run CI on containers of various distros FWIW https://github.com/nexB/scancode-toolkit/blob/c89e8b39d358befb90cd306de747b9086e71a7a9/azure-pipelines.yml