Change 'python' to 'python3' everywhere.

Sigh.

Depending on the system and environment, python can mean python2 or python3. PEP 394 recommends using python3 wherever you need to be explicit. Yes, in many environments (conda python=3; some new OSes) we can simply use python and always get python3. But that is not robust. Or we could maintain backward compatibility "forever", which we really do not want to do.

Shebangs, various eups, MHS, ics_config, system scripts. This is related to INSTRM-279, which is for the ansible side of things.

Deep sigh.