Fill PYTHON2 and PYTHON3 variables. Allow override of everything.
Favor the generic version name because we expect the "better"
python2/python3 version to be set by the distribution.

This is starting to get quite repetitive though.

Always run Python scripts through the same interpreter.
Also simplify the running of pyflakes and flake8, as recent versions all
support execution through python -m. Due do the many variations among
distributions, it makes the whole thing much simpler.

This should make running tests faster, as we do not remove all .pyc
files anymore!