Comment 2 for bug 1853809

Thanks Kenneth, I've tried to backport your fixes to the Ubuntu package but it still fails to build

https://launchpadlibrarian.net/453033403/buildlog_ubuntu-focal-amd64.duplicity_0.8.07-1ubuntu2~build1_BUILDING.txt.gz

The _librsync import error are weird though which makes me wonder if that's something in the packaging setup (though the same package builds fine on 19.10 with python 3.7)