Message3272

it looks like we end up exiting because we actually write our prompt 
'>>>' to sys.stdout. So writing to the closed stdout raises a 
ValueError, breaking us out of the interactive prompt loop (causing the 
exit)

We should actually be writing the prompt to stderr -- that will totally 
avoid the exception.

The problem with that is when readline is enabled, the only way we can 
write the prompt to stderr instead of stdout is by doing it ourselves 
(right now Readline.readline() writes the prompt for us).

This seems to break readline in some situations: for example, when 
recalling a previous line in the history (with the up arrow), readline 
for some reason doesn't realize there's a prompt at the beginning of the 
line and allows you to muck with those characters