With sympy==1.5 kwant.qsymm.builder_to_model results in malformed BlochModel
BlochModel returned by
momenta as strings, while these should be
sympy.Symbols. The reason is
all(m in qsymm.model._commutative_momenta for m in ('k_x', 'k_y')) (where
_commutative_momenta is a list of
sympy.Symbols) evaluates to True in 1.5, it is False in all other versions I tried. We should require
sympy==1.5.1 for example, or rewrite this check in some more failsafe way.