Switch from python2 to python3
A bunch of work has been made since 0.7 that should make python parts work with both python 2.7 and python 3. This should allow us to switch over to python3.
This includes renaming the python-avahi package to python3-avahi. The only remaining package with a hard dependency on python-avahi is avahi-discover, which is also fixed in this commit.
[smcv: squash 0beec518, 5cb43f9c, 56b02b8b into one commit]
Closes: #853239 (addition of python3-avahi)
Closes: #936173 (removal of python-avahi)
Separated from the new upstream release in !1 (merged) for easier review. We should probably release v0.8 first, and then put this change through NEW.