说ord（some_buffer [n]）的可移植方式是什么？

Question

> python2 -c "print(ord(b'a'[0]))" 
97
> python3 -c "print(ord(b'a'[0]))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: ord() expected string of length 1, but int found

如何以适用于Python 2和Python 3的方式编写此代码？