Charm tests require Ubuntu 16.04 (Xenial) and Python 3.5, both of which are EOL

[barryprice]

Jenkins and plugins versions report

Not relevant

What Operating System are you using (both controller, and any agents involved in the problem)?

Ubuntu 22.04

Reproduction steps

1. Download the charm source
2. Run make

Expected Results

All bundled tests should run as expected on the latest Ubuntu LTS

Actual Results

SKIPPED: InterpreterNotFound: python3.5
_____________________________________________________________ summary _____________________________________________________________
SKIPPED:  py35: InterpreterNotFound: python3.5
  congratulations :)

Anything else?

The tests all reference "xenial" or "Xenial", with nothing set up for newer, supported series.

I'd expect to see first class support for testing on 22.04, 20.04 and 18.04 in that order, with 16.04 support a bonus.

Latest LTS (22.04) uses Python 3.10, Focal (20.04) uses Python 3.8 and Bionic (18.04) uses Python 3.6 - the tox config should probably support some or perhaps even all of these.

Requiring Xenial (16.04) is particularly problematic now that deploys using the LXD provider on modern Ubuntu no longer support networking on 16.04 without disabling cgroup2, impractical for many (most?) devs:

https://github.com/lxc/lxd/issues/9413