Get child nodes which has ``class_name`` class attribute.

def getElementsByClassName(start_node: ParentNode, class_name: str
                           ) -> NodeList:
    """Get child nodes which has ``class_name`` class attribute."""
    classes = set(class_name.split(' '))
    return getElementsBy(
        start_node,
        lambda node: classes.issubset(set(node.classList))
    )