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

See also https://github.com/nexB/skeleton/issues/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:
- [x] 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
- [x] drop Python 3.6 everywhere
- [x] move to Python 3.8 https://github.com/nexB/scancode-toolkit/issues/2664
- [x] add Python 3.10.0 https://github.com/nexB/scancode-toolkit/issues/2730