code: def url(self, node):
          
          a_node = node.findParent('a')
          if not a_node:
              return None
  
          if self.link_callback is None:
              return a_node.get('href')
          else:
              return self.link_callback(a_node.get('href'))
docs: return the url of a BeautifulSoup node or None if there is no
          url.