Python 3 fix for 'setup.py'.

codinthehole/purl/master@acc683b06e19f134377bf12256537dbfcb8d8cd3 broke `setup.py` for Python 3. This commit adds an helper function (based on [six](https://bitbucket.org/gutworth/six)) that should help with future cases of unicode string literals that need to be byte string for Python 2.