I think that your patch works only for terminals where one byte of the 
encoded text is displayed as one character on the terminal. This is not 
true for utf-8 terminals, for example.

In the attached patch, I tried to write some unit tests, (I had to adapt 
the traceback module as well), and one test still fails because the 
captured stderr has a utf-8 encoding.
I think that it's better to count unicode characters.