continuum: substituting pauli matrices doesn't work inside kron
For example this work
kwant.continuum.sympify('tau_x', locals={'tau_x': 'sigma_x'})
but this throws an exception:
>>> kwant.continuum.sympify('kron(tau_x, sigma_y)', locals={'tau_x': 'sigma_x'})
AttributeError: ImmutableMatrix has no attribute args_cnc.