Calculates the lowpoints for a single node in a graph.

def __get_lowpoints(node, dfs_data):
    """Calculates the lowpoints for a single node in a graph."""

    ordering_lookup = dfs_data['ordering_lookup']

    t_u = T(node, dfs_data)
    sorted_t_u = sorted(t_u, key=lambda a: ordering_lookup[a])
    lowpoint_1 = sorted_t_u[0]
    lowpoint_2 = sorted_t_u[1]

    return lowpoint_1, lowpoint_2