run 2to3 on documentation

doc/source/conf.py

doc/source/tutorial/ab_ring.py

doc/source/tutorial/band_structure.py

doc/source/tutorial/closed_system.py

doc/source/tutorial/graphene.py

doc/source/tutorial/quantum_well.py

doc/source/tutorial/quantum_wire.py

doc/source/tutorial/quantum_wire_revisited.py

doc/source/tutorial/spin_orbit.py

doc/source/tutorial/superconductor_band_structure.py

doc/source/tutorial/superconductor_transport.py