Python runner in Python3 mode can't make use of the virtualenv option as provided because that module DOES NOT EXIST! Python3 renamed it to `venv`. Mixing python2 env with python3 runtime is ill advised and we dont desire to have any python2 on our host where possible to avoid issues.
If the python environment is specified as python3 the options should be corrected to venv instead of virtualenv for the command line switch.
Moreoever it seems like an additional instruction is required that you must add a config variable of python3.executable=/usr/bin/python3 or the build agent wont recognize that it has python3 available to it even if it is in the PATH. This was completely overlooked and skipped over in the python runner demo video.