Comment 7 for bug 1977888

We could pin train to an older dib, but we need to consider the likelihood that train will need a dib fix in the future which it can't get without a stable branch.

How about the periodic-tripleo-centos-8-buildimage-overcloud-full-train job install and use python3.8? One way would be a task which detects if default python is < 3.8, installs 3.8, then switches the default to it using "alternatives"