It's working with Python 3.6.9 on Linux Mint..
An updated version Leo 6.6.2 is now available on GitHub and pypi. The updated version runs on Python 3.6. The previous version didn't.This will be the official 6.6.2 version unless truly serious problems arise. We'll fix minor problems in the 6.6.3 cycle. Nevertheless, I'll wait until Friday to announce 6.6.2 publicly.
I've tested the combination of 'devel' at GitHub & '6.6.2-2' at PyPI in a Debian 11 & a Fedora 34 VM.The 'test-all' command is working everywhere.