grep -v only fails if there are no unmatched lines, but that's not what we want to test.
See https://lists.fedoraproject.org/archives/list/packaging@lists.fedoraproject.org/thread/UFKUM5UKCTNGIT3KJVYEI5VXPI23QMBN/ Flatpak builds redefine %_prefix and the dependencies should remain present. Also get rid of one useless ^ and prep the pattern for two digit Python major versions. Add a test that tests that we match our default %_prefix (was the case even before this commit).