more pep8 / pep257 cleaning of Graph.py (done)
more pep8 / pep257 cleaning of Graph.py (done)

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