Add check for py3 compatibility

Changes introduced

Add check in check_jo_consistency CI job to check if a jO is python3 compatible.

The CI job runs in python3 so this only makes sense for checking if old jO are py3 compatible (but not if a jO is also py2 compatible). I wonder if we want to implement both.

Tests

Local

CI

• With problematic file provided by Matthew: test fails as expected: https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/21235798
• Test py2 syntax with print function from __future__ - test passed as expected: https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/21236879 and I could run the jO with both r21 and r22

Issues resolved

Closes #166 (closed)