Run Sphinx doc generation in Python 3

[Hexcles]

We use Sphinx to parse arguments from a few wpt commands and generate docs automatically. It's currently running in Python 2. This is a reminder that once we switch our primary supported version to Python 3, we should upgrade Sphinx and run it with Python 3.

[Hexcles]

Code is in https://github.com/web-platform-tests/wpt/tree/master/docs

[stephenmcgruer]

@ziransun - can you start looking at how compatible our document generation is with Py3? (Low priority, but since the RFC is ongoing).

I believe wpt build-docs will run it, but you'll need to make sure that it's actually using py3 all the way down. Very high level list of things I think will likely need to happen:

1. Make sure that Python3 is being used all the way down when running wpt build-docs.
2. Update to a new Sphinx version (in docs/requirements.txt) that supports py3
3. Figure out any problems, iterate on fixing them :)
4. Eventually update the docker image used for this (see tools/docker/documentation/Dockerfile) to be py3 based, though this shouldn't be required for local testing.

@Hexcles may have more thoughts.

[ziransun]

Thanks @stephenmcgruer! I will take a look.