How to use the parsel.Selector.selectorlist_cls function in parsel

def test_extending_selector(self):
        class MySelectorList(Selector.selectorlist_cls):
            pass

        class MySelector(Selector):
            selectorlist_cls = MySelectorList

        sel = MySelector(text=u'<div>foo</div>')
        self.assertIsInstance(sel.xpath('//div'), MySelectorList)
        self.assertIsInstance(sel.xpath('//div')[0], MySelector)
        self.assertIsInstance(sel.css('div'), MySelectorList)
        self.assertIsInstance(sel.css('div')[0], MySelector)

__all__ = ['Selector', 'SelectorList']


def _st(response, st):
    if st is None:
        return 'xml' if isinstance(response, XmlResponse) else 'html'
    return st


def _response_from_text(text, st):
    rt = XmlResponse if st == 'xml' else HtmlResponse
    return rt(url='about:blank', encoding='utf-8',
              body=to_bytes(text, 'utf-8'))


class SelectorList(_ParselSelector.selectorlist_cls, object_ref):
    """
    The :class:`SelectorList` class is a subclass of the builtin ``list``
    class, which provides a few additional methods.
    """


class Selector(_ParselSelector, object_ref):
    """
    An instance of :class:`Selector` is a wrapper over response to select
    certain parts of its content.

    ``response`` is an :class:`~scrapy.http.HtmlResponse` or an
    :class:`~scrapy.http.XmlResponse` object that will be used for selecting
    and extracting data.

    ``text`` is a unicode string or utf-8 encoded text for cases when a