Backport to Python2

Backport xlte to py2 so that at least tests pass and that kpidemo.py and
kpidemo notebook work. This means that we can parse and process logs on
older systems where we have only python2 available.

I did not verified in detail whether xlog generation continues to work
ok. But that is barely needed - we generate xlog from newer systems
running on ORS(*) and similar, and we need python2 support only to
process enb.xlog somewhere else.

(*) https://www.rapid.space/products/ors