oslo.log exceptions with py39

[Sorin Sbarnea]

Apparently stestr lacks support for py39 as we can see exception in the logs but at the same time the exit codes seems to be still success, which is a double problem. I would argue that ignoring exceptions from the test tools can be see as even more dangerous as it may give us some false positives.

Example: https://review.opendev.org/c/openstack/tripleo-common/+/780573

py39 jobs contains some exceptions that are not seen on the other jobs like http://paste.openstack.org/show/803566/