Bump debhelper compat version from 11 to 12

[sssoleileraaa]

Description

When we bump the debhelper compat version from 11 to 12, CI for workstation components break. We are able to repro by first creating the tarball in the Python project directory via setup.py, and then telling the deb packager to create a deb package using that source tarball. This is a feature we've long supported, and haven't desired removing, as far as I know. Let's investigate what change between 11 and 12 that would cause this to break.

[legoktm]

This is an upstream issue with dh-virtualenv: spotify/dh-virtualenv#326 and one of the reasons I've proposed dropping it altogether: freedomofpress/securedrop#6538.

[legoktm]

Now that we've dropped dh-virtualenv, bumping to 12 should be doable.