debuild: skip 'make check' and fix the broken dh-python dependency.

These are two quick fixes to make the OBS Debian/Ubuntu builds work again.

The first one works around a bug in the wake of !524 (merged), which makes the dh_auto_test target in debuild/debian/rules fail. Here's the default dh_auto_test target again, trying to outsmart its human master, deciding that it wants to run make check which can't possibly work in this case, because I modify the staging area quite heavily in the Debian builds so that purr-data and pd-l2ork1 can coexist on the same system. This wouldn't be the case previously, simply because we had no make check target. In fact I should have anticipated this, and modified that darn rules file proactively, but I didn't. ;-)

The second one fixes up some dependencies which have been broken on Debian Unstable and Testing for a while now, in a manner that the builds continue to work on stable and old Debian/Ubuntu releases.

The required changes are strictly confined to the debuild/debian/control and debuild/debian/rules files, so they won't affect the CI, make and tar_em_up.sh in any way. I already tested the change on the OBS and it works, i.e., all JGU packages now build fine again.

Jonathan, it would be really important to have this in 2.14.1, so that I don't need to patch the source in order to get the OBS builds working. The CI can't possibly test this anyway, so it would be nice if you could just fast-track it and merge into master right away. Thanks.