PyChem is part of GEAR. GEAR may be downloaded using the open source version control system git:
git clone ssh://obslogin1.unige.ch/www/people/revaz/public/git/gear gear
To install PyGear:
cd gear/PyChem/
python setup.py install
or if you want to install it in a particular directory
python setup.py install --prefix /your/prefered/directory