In Tutor core & plugins, the minimum Python version should be bumped from 3.8 to 3.9

[regisb]

Is your feature request related to a problem? Please describe.

Python 3.8 is reaching end-of-life on Oct. 31st: https://endoflife.date/python This means that the minimum supported version of Tutor should bump to 3.9.

Describe the solution you'd like

1. we should bump the minimum required version of Tutor and all plugins to 3.9.
2. CI must be upgraded to build binaries with 3.9.
3. We must get rid of documentation warnings triggered by python 3.8
4. Basically, all references to 3.8 must be removed.