tests/packaging: deal with pkg_resources removal
This issue doesn't seem urgent, but we see it in CI and we'll need to deal with it at some point
https://gitlab.nic.cz/knot/knot-resolver/-/jobs/2008287#L1153
https://setuptools.pypa.io/en/stable/history.html#v82-0-0
```
$ git grep pkg_resources | cat
tests/packaging/dependencies.py:import pkg_resources
tests/packaging/dependencies.py:installed = {pkg.key for pkg in pkg_resources.working_set}
```
task