Find Linux package dependencies

Numerous Python packages use PyTest extensively. Upon apt install a package that depends on python-astropy, python-pytest is also installed. The system version of a package is typically several minor versions behind for stable Linux distros like Ubuntu. To find which specific package is responsible for a package being installed, use a command like:

apt-cache rdepends --installed python-pytest

which reveals that

python-py
python-astropy

are requiring this old pytest version.