Convert jo_utils to python2 so that it can run in old releases

Description of bug

In old releases python3 is not available and run_athena and everything that relies on the python3 commands in jo_utils.py crashes

Changes introduced

Convert python3-commands to python2-commands in jo_utils.py and run_athena.sh

Tests

See test pipeline https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/pipelines/4016451
512998: py3 release
512999: py2 release
They both run

Issues resolved

Closes #176 (closed)