Upgrade default python version from python3.8 to python3.9

Python3.9 is now the default version of Python3.

Python3.10 can't be the default Python because of several non compatible packages (like pygolang).

• depends on https://lab.nexedi.com/nexedi/slapos/merge_requests/1279
• depends on https://lab.nexedi.com/nexedi/slapos/merge_requests/1280