Debian has removed the python-all package from its Bookworm repos, which
breaks our CI tests. Looking into why python-all is required in the
first place, we found that it's an artificial stdeb requirement [1],
prior to 0.9.1 versions
The only platform affected by this issue is Ubuntu Focal, so our
solution is to install python-all specifically for that platform.
Finally, we further simplify our build tasks [2] (on Debian-like
distros) by not letting dh-python run tests when building the packages.
Running the tests has some issues after all:
1. It requires installing all the runtime dependencies of Dangerzone,
since it uses `python -m unittest discover` underneath.
2. It doesn't aid in the stability of the package, since unittest cannot
run test cases for PyTest.
[1]: https://github.com/astraw/stdeb/issues/153
[2]: https://github.com/freedomofpress/dangerzone/issues/292#issuecomment-1349967888
