Make OneTime source code compatible with both Python3 and Python2.
This was originally https://github.com/kfogel/OneTime/pull/4 . It's closed now -- in the end, I decided just to go to Python 3 and not try to keep Python 2 compatibility.
I'm reflecting all the GitHub OneTime issues over here at code.librehq.com, preserving ticket numbers. For closed tickets like this one, I'm not bothering to copy over any of the details. (There's a way to import them all properly into GitLab, but there aren't enough issues to make it worthwhile to learn how to do that. Instead, I'm just copying them over by hand.)