Failure of attach_lead on master
To reproduce:
lat = kwant.lattice.honeycomb()
syst = kwant.Builder()
syst[lat.sublattices[0](0, 1)] = syst[lat.sublattices[1](0, 0)] = 0
lead = kwant.Builder(kwant.TranslationalSymmetry(lat.vec([-1, 2])))
lead[lat.sublattices[0](0, 0)] = lead[lat.sublattices[1](0, 0)] = 0
lead[lat.sublattices[0](0, 1)] = lead[lat.sublattices[1](0, 1)] = 0
lead[lat.neighbors()] = 0
kwant.plot(syst)
kwant.plot(lead)
syst.attach_lead(lead)