Build pyzfs package (and other housekeeping)

This 3 part patch-set follows upstream's changes to default to python3, builds the python3 version of the new pyzfs library, and fixes a build failure due to the migration from zfs-dbg to the automatic dbgsym packages (by removing an unneeded option passed to dh_strip).

The build should work with dh_missing --fail-missing, and I'd recommend setting that again by reverting c5024683.