Update Python 3.8 -> 3.?

[ricklentz]

Can we bump the Python version (at least to 3.9)?

[jrobble]

Thanks for your interest in OpenMPF! This will be addressed by openmpf/openmpf#1687 when we update our builds to the newest Ubuntu LTS. We've got some large features we're working on for release 10.0 that come before that, so it's likely still a few months away.