docs: def popitem(self):
          """Removes the first key-value pair and return it.
  
          This method raises a ``KeyError`` if the list is empty.
          """
          node = self._head[2]
          if node is self._tail:
              raise KeyError('list is empty')
          self._find_lt(node[0])
          self._remove(node)
          return (node[0], node[1])