fix linter errors everywhere

qsymm/__init__.py

qsymm/groups.py

qsymm/hamiltonian_generator.py

qsymm/kwant_linalg_lll.py

qsymm/model.py

qsymm/symmetry_finder.py

qsymm/tests/test_groups.py

qsymm/tests/test_hamiltonian_generator.py

qsymm/tests/test_model.py

qsymm/tests/test_mutual.py

qsymm/tests/test_symmetry_finder.py

qsymm/tests/test_util.py