markdown python，exercise，iterator

Posted 2021-05-06

when use python in the way below, there will be an exception: 
```
In [19]: it = iter([1,2,3])
In [20]: it.next()

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-20-24d6ab57088b> in <module>()
----> 1 it.next()

AttributeError: 'list_iterator' object has no attribute 'next'
```
this is a syntax change between python2 and python3. in 3 you can use next(it) instead.