fix string types for py3
fix string types for py3

yingerj/rdflib,RDFLib/rdflib,armandobs14/rdflib,dbs/rdflib,RDFLib/rdflib,marma/rdflib,armandobs14/rdflib,avorio/rdflib,armandobs14/rdflib,yingerj/rdflib,ssssam/rdflib,ssssam/rdflib,marma/rdflib,marma/rdflib,RDFLib/rdflib,ssssam/rdflib,avorio/rdflib,dbs/rdflib,dbs/rdflib,armandobs14/rdflib,avorio/rdflib,avorio/rdflib,ssssam/rdflib,dbs/rdflib,RDFLib/rdflib,yingerj/rdflib,yingerj/rdflib,marma/rdflib