text: Title: Add support for pypy3
  Body: The PyPy team has added support for Python 3 (see https://morepypy.blogspot.it/2017/03/pypy27-and-pypy35-v57-two-in-one-release.html).
  
  We should add this to our test matrix:
  - In the tox.ini file
  - In the travis.yml test setup
  - In the setup.py "classifiers / implementation" section
  - In the README.rst
  
  Implementation should be:
  1. Add those changes
  2. Make a pull-request
  3. Check that all goes fine
  4. Add yourself to the CREDITS file if this is your first contribution
  5. Enjoy!