tox: Fix install, add pypy.
tox: Fix install, add pypy.

dforste/pypuppetdb,jcastillocano/pypuppetdb,voxpupuli/pypuppetdb,jorik041/pypuppetdb,jcastillocano/pypuppetdb,puppet-community/pypuppetdb,amwilson/pypuppetdb,vicinus/pypuppetdb