The default compression method used by dpkg in Sid, Jessie, (and Trusty Tahr) is now xz (changed from gzip)
So tools that write to .deb using dpkg defaults and read using python-debian are now broken (python-debian will throw an exception).
-- http://lists.alioth.debian.org/cgi-bin/mailman/listinfo/pkg-python-debian-maint
