Python 3 port

It seems there are several dependencies that do not support Python 3, which prevents us from running anything in Python 3. Here is a verification tool that I used: https://caniusepython3.com/check/77d09f52-3454-4171-9a3d-a27f2ed683d8

There was also this helpful talk at PyCon, which mentions some useful tools like python-modernize and futurize: https://www.youtube.com/watch?v=KPzDX5TX5HE
