[visitor] Implement iterator.__next__()

1          self._next = [(node, 0)]
2          if filter_func is None:
3              filter_func = no_filter
4          self._list = list_func(node, filter_func)
5 
6 -    def next(self):
7 +    def __next__(self):
8          try:
9              return self._list.pop(0)
10          except :
11              return None
12 
13 +    next = __next__
14 +
15  # Base Visitor ################################################################
16  class Visitor(object):
17 
18      def __init__(self, iterator_class, filter_func=None):
19          self._iter_class = iterator_class