Include yapf or black in linting

[jreiberkyle]

The results from yapf using the default style, pep8, seem pretty reasonable. Black is another popular option. Black is not configurable, yet yapf may be non-deterministic (Hacker News thread).

Chose which to use, then run it on the planet and tests directories. Add it to the automatic linting for the project, and make reference to it in the CONTRIBUTING doc.

[jreiberkyle]

Increase priority, re: #403 (comment)

[jreiberkyle]

yapf is closest to the existing style, so that's what we will use