Merge branch 'python3'

doc/Makefile

doc/source/conf.py

doc/source/images/graphene.py.diff

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

examples/3d_plot.py

examples/advanced_construction.py

examples/logo.py

examples/square.py

examples/tests/test_square.py

kwant/__init__.py

kwant/_common.py