Default python version must remain 2.x even on translate.lizard

Originally created by @intrigeri on #16152 (Redmine)

$ python --version
Python 3.5.3

Debian did not do this transition yet; AFAIK no distro has, actually; lots of Python stuff is not ready for it, e.g. it breaks rdiff-backup.

This change seems to have been applied by hand (i.e. not via Puppet) on Nov 10. Please revert it and ensure whatever need prompted this change is addressed in a nicer manner :)

Parent Task: tails/sysadmin#11759