That is a Python package that is maintained by the Debian repo. You can install it with the Debian package manager:
sudo apt-get install python-pythonmagick
or
sudo aptitude install python-pythonmagick
However, I would prefer to stick with pip (with virtualenv) and use either of the following instead:
→ pip search pythonmagick
PythonMagickWand - Python wrapper for the ImageMagick wand API
PythonMagick - Wrapper around GraphicsMagick
→ pip search pgmagick
pgmagick - Yet Another Python wrapper for GraphicsMagick
Note that you will need to install GraphicsMagick/ImageMagick itself with Debian's package manager separately if you are using pip. pgmagick/pythonmagick is just a Python wrapper for GraphicsMagick/ImageMagick.
More: pgmagick