Migrate to Python 3

[psvenk]

Closes #38. This is very much not yet ready to merge. This seems to be working from some preliminary testing, but more testing is needed.

Some changes correspond to changes between Python 2 and Python 3 (e.g., import semantics, stdlib package layout, indentation enforcement); others correspond to changes in Django. I also moved the requirements to a requirements.txt file and removed the old version numbers (the listed Django version is incompatible with modern versions of Python anyway).

[cjquines]

is fireroad-server run on its own server, or somewhere else? athena and scripts, for example, have a quite outdated version of python3

[psvenk]

This should be pretty close to ready to merge now. I tested with CourseRoad, Hydrant, and catalog updates, and the Python 3 version exhibits identical behavior to the Python 2 version (on master) in all cases. The test suite still fails on five cases, but this is not a regression.

[venkatesh-sivaraman]

Awesome work @psvenk!

The server runs on an IST managed server which should have a Python 3 installation (IIRC there's a virtualenv defined for FireRoad). Have you tested this on the dev server yet? If not I'd definitely recommend doing that before merging, since I suspect database migrations with Django will be a headache. You might want to even make notes of the steps you take to set it up on dev, to make the process smoother on prod. (I believe I tried to do a Python 3 conversion a while back, and ran into too many roadblocks for it to be worth continuing, which is why I mention this!)

Also, if all of that goes well, you might consider reaching out to the IST team to let them know you're making this change and to make sure there is an available backup of the prod database :)