Finalize Python 3
I based this branch on the existing python3 branch, which I rebased against master.
Porting to Python 3 was done with '2to3'. I also included two small modifications to moz_version.py. These changes is in separate commits.
Rebuilt tree-style-tab and dispmua, controlled with diffoscope and found no change. These two packages are the only ones still left in testing.
I am unsure about the version number, I would assume a major version bump.