Add more Python versions to cloud-init's Travis CI

[Dan Watkins]

Currently we run cloud-init's unit tests twice in Travis:

* once on Python 3.6 with the latest versions of our dependencies from PyPI, and
* once on Python 3.5 with the versions of our dependencies that are in xenial (but installed from PyPI, rather than apt)

We currently know that cloud-init is run on 3.7 and 3.8, because eoan and focal use those versions respectively.

We should expand our matrix of testing to cover all currently-supported Python versions, 3.4+.

[Pat Viafore (patviafore)]

Would it also be worth running in Python nightly (maybe not blocking the build on that, but I know that's caught things for me - I know some things like collections.abc vs collections is going to impact python 3.9/3.10

[Dan Watkins (oddbloke)]

Yep, we could add a job that uses the "nightly" Python version, or Travis has a "$version-dev" (e.g. 3.9-dev) which might be more stable than "nightly" (or might just be what "nightly" points at, I'm not sure). See https://docs.travis-ci.com/user/languages/python/#specifying-python-versions for the details.

(And, yes, we would want these to be non-voting tests as they aren't running against a released Python version.)

Thanks for the suggestion, Pat!

[James Falcon (falcojr)]

We're currently running up to 3.12-dev in Travis, which is the current development version, so this bug is no longer relevant.
https://github.com/canonical/cloud-init/blob/main/.travis.yml

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3661