警告内容原话：

FutureWarning: The behavior of this method will change in future versions. Use specific 'len(elem)' or 'elem is not None' test instead.
  if next_page_a:

意思就是将 if next_page_a:替换掉，
我的替换成 if next_page_a is not None:或者if len(next_page_a) > 0:就可以了