fix formatting

examples/square.py

kwant/builder.py

kwant/digest.py

kwant/graph/tests/test_scotch.py

kwant/graph/tests/test_utils.py

kwant/lattice.py

kwant/linalg/lll.py

kwant/rmt.py

kwant/system.py

kwant/tests/test_builder.py

kwant/tests/test_lattice.py