This commit:
1. Formats all of our existing code using `black`.
2. Adds a note about using `black` to `CONTRIBUTING.rst`.
3. Runs `black --check` as part of CI (failing builds that aren't properly formatted).
1. Removes the cascade=save tests. It's not an option I'd recommend using AND
it primarily matters if you have any reference fields in your document,
which is not the case in this script.
2. Uses PyMongo-v3.x-style write concern.
3. Removes an old docstring describing some random benchmark run from the past.
4. Removes unused parts of the code.
I'll add more tests to the "benchmarks/" directory in future commits.
