Drop python2 support

Remove all compatibility code for python2.

All scripts are already python 3 compatible since
I80e5850a8978d78cda793e2192ef4bd3fd54a121 and
I1b4a629f12863c498a8681b555f57b4e255cebfb.

dpkg-buildpackage shows that it is still invoking setup.py with python
in addition to python3, on debian stretch. But after spending quite some
time on trying to convince it to not care about python2 without success
(trying different variables, overrides, --without python2 flags etc.),
I'm leaving it as is. The resulting package is the python3 package, which
is what we want.

Related: OS#2819
Change-Id: Iabda95073faa2191fd117e9637e0858c589e9d9e
13 files changed