Python 2.7 compat until end of 2018

I know, this sounds horrible, but we have huge troubles with ROOT6, rootcling and Python 3.6 (the admins in Lyon could not manage to get ROOT6.06 with Python >3.5 running, due to an Python 3.6 incompatibility bug in ROOT), so we may consider to support Python 2.7 until the end of 2018. Absolutely no longer than that, but we need time for a smooth transition.

Again, sorry, I will take care of this (I am pissed as hell)...