Port debsums test from perl to Python

This runs on all our Debian and Ubuntu machines to catch cases where people have edited files that are under the control of the package manager. In the best case those files will get overwritten by future package updates; in the worst case a changed file can be the sign of a serious problem.

Adam wrote:

https://gitlab.developers.cam.ac.uk/ch/co/kernel-decompresser will have to be updated when this gets released : it writes perl config fragments when kernels are installed. And https://gitlab.developers.cam.ac.uk/ch/co/ansibleconf/-/blob/master/roles/domu/tasks/pygrub_kernel.yml#L91 is in a similar vein.