$ sudo apt-get update
$ sudo apt-get install python-pynzb 

python-pynzb is installed in your system.

Make ensure the python-pynzb package were installed using the commands given below,

$ sudo dpkg-query -l | grep python-pynzb *

You will get with python-pynzb package name, version, architecture and description in a table

python-pynzb package basic information: