We do this to take advantage of python 3.10's speed improvements as
illustrated by Zuul proper. But also OpenDev would like to drop python
3.8 image builds to make room for python 3.11.
We add python3.10 unittesting to prevent regressing the docker images on
that version of python.
Change-Id: Ib2e39e0c5d09b4fdce40faa072fe51b55c8d7407
c338fc970d