run 2to3 on Kwant codebase

kwant/__init__.py

kwant/_common.py

kwant/builder.py

kwant/digest.py

kwant/graph/__init__.py

kwant/graph/tests/test_core.py

kwant/graph/tests/test_dissection.py

kwant/graph/tests/test_scotch.py

kwant/graph/tests/test_slicer.py

kwant/graph/tests/test_utils.py

kwant/lattice.py

kwant/linalg/__init__.py

kwant/linalg/decomp_lu.py

kwant/linalg/fortran_helpers.py

kwant/linalg/lll.py

kwant/linalg/mumps.py

kwant/linalg/tests/_test_utils.py

kwant/linalg/tests/test_linalg.py

kwant/linalg/tests/test_lll.py

kwant/linalg/tests/test_mumps.py