In RHOS-15, we are shipping python-stestr-2.1.0 which is quite old. We need a newer version of python-stestr i.e. 2.3.1 for tempest. It has lots of improvements and needed for running tempest on RHOS-15 using python3. It is already built in RDO: https://cbs.centos.org/koji/buildinfo?buildID=26172
python-stestr-2.3.1-3.el8ost is present in the rhos-15 repo available also via latest symlink.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHEA-2019:2811