Make python3 default for build script

[ischoegl]

Abstract

Auto-detect python version to ensure compilation with python 3 as default.

Motivation

Avoid having to specify

python_cmd = 'python3'

in cantera.conf (or use option scons build python_cmd=python3) for systems that have both versions installed

PS: now that python 2 has reached end-of-life (see sunset-python-2), it appears more appropriate to 'opt out' of using python 3 for the few cases where it is necessary (server installs?).