Add six != 1.9 to the requirements. According to Björn, the version of six in Debigan wheezy is too old and odfcast needs at least version 1.9. For some reason it was missing from requirements.txt, so we add it now.

flask>=0.7
requests
py3o.template>=0.8
py3o.renderers.pyuno>=0.3
setuptools
pypdf2
six>=1.9