Add python 3 support

[tstapler]

Hi, Thanks for all of the great work on this tool. I use it daily.

Since python 2 was removed from homebrew, it's been a pain to keep using repren. I had some time and I thought I would try my hand at a python 3 port. #19

I used a combination of the 2to3 tool and manually converting all operations to explicitly use binary. It turns out it wasn't too hard.

The functional tests are the primary way that I tested the changes. Let me know if you'd like any changes.