So 3.10.0 and 3.10.1 now work (with Bionic and Focal), that’s good news.
But 3.10 still does not work because python-3.10.tar.bz2 is lacking, as shown in this list https://docs.travis-ci.com/user/languages/python/#python-versions.
This tar file needs to be added just like python-3.9.tar.bz2, python-3.8.tar.bz2 are already present next to their minor releases.