Debian/Ubuntu: compat level 13 breaks newer builds

Compat level 13 (as forced by the build-dep on debhelper-compat (= 13)) for recent distros (in particular, Trixie) introduces new default behaviour of dh_missing, erroring out on files left below .pytest_cache but "not installed to anywhere". This results in build breakage.

There are multiple ways to fix it as already discussed for ciecplib here. I'm in favour of adding a dh_missing override, but @duncanmmacleod may prefer something different for the next release?