docs: def sort(self, *args, **kwargs):
          """Sort the MultiMap.
          
          Takes the same arguments as list.sort, and operates on tuples of
          (key, value) pairs.
          
          >>> m = MutableMultiMap()
          >>> m['c'] = 1
          >>> m['b'] = 3
          >>> m['a'] = 2
          >>> m.sort()
          >>> m.keys()
          ['a', 'b', 'c']
          >>> m.sort(key=lambda x: x[1])
          >>> m.keys()
          ['c', 'a', 'b']
          
          """
          self._pairs.sort(*args, **kwargs)
          self._rebuild_key_ids()