Internal error if weights aren't a list

[BenWeber42]

This snippet will cause an internal error (AttributeError) because dusk always assume that weights will be a list:

@stencil
def test_non_list_weights(a: Field[Edge], b: Field[Vertex]):
    with levels_upward:
        a = sum_over(Edge > Vertex, b, weights=5)

A negative test should be created for that once the testing framework supports negative tests.