Update python-apt in byzantium
Another try for updating python-apt to 2.2.1 from Debian testing.
Merging the Debian modifications back into PureOS and rebuilding the tree afterwards in a byzantium chroot works without problems. Also the autopkgtest working (of course). But I haven't done further PureOS specific testing simply because I don't have currently an idea how to do that.
I'm open for possible steps and suggestions to doing better on that.