sudo apt-get install python-catkin-pkg
The Debian package only contains the command line scripts and depends on another package named python-catkin-pkg-modules which contains the Python modules required for the scripts. This allows to install python-catkin-pkg-modules and python3-catkin-pkg-modules at the same time.
On non ubuntu platforms
sudo pip install -U catkin_pkg
In case you experience error with the command above, you might want to run it without -U option. See this thread.