* Removed unused `.package`-option.
* Added explicit postgresql support.
* Create a new meta-package for mailman to make sure each component has
the **same** python and packages can be downgraded if needed (e.g.
psycopg2 or sqlalchemy) without interfering with `pythonPackages` in any way.
* Document why certain python overrides are needed.
Closes#170035Closes#158424
