make use of "as_ordered_terms" in "_extract_hoppings"

kwant/continuum/discretizer.py

@@ -405,7 +405,7 @@ def _discretize_expression(expression, coords):
     def _extract_hoppings(expr):
         """Read hoppings and perform shortening operation."""
         expr = sympy.expand(expr)
-        summands = expr.args if expr.func == sympy.Add else [expr]
+        summands = expr.as_ordered_terms()
 
         offset = [_read_offset(s.args[-1]) for s in summands]
         coeffs = [sympy.Mul(*s.args[:-1]) for s in summands]