* automatically find the chroot binary on Debian, even if it is not
in the user's PATH
* don't use subprocess.run anymore (remove related testcase, that explicitly
checked for subprocess.run usage, and used recursive globbing, another
post 3.4 Python feature, for the checks. A similar case can be added in the
future, but right now it's more important to get Debian 3.4 working and all
PRs are reviewed anyway.)
* pytest fixtures: don't use the newer "yield" feature, as this is only
supported in a newer version of pytest, than provided on Debian Jessie
From manually testing, most stuff works in Debian Jessie. However, the
testsuite does not run through - creating an empty .tar.gz with Python
fails for some reason (this is done in test_apk_static.py).