Drop python2 support; Closes: #937691

This is pretty straight-forward, i dont have access to the official git repo so i'm opening this MR, but it would like to proceed quickly, as python-debian is now the only remaining rdep of python-apt.

Ideally, i would like this to get merged and released in the next couple of days (I can do a team upload if you add me to the salsa team, there's no button to request access to the team) so that we can then drop python-apt

thanks!