Change autoformatter from pyformat to Black

[Zac-HD]

Because Black is a better autoformatter than autopep8, which pyformat uses under the hood. pyformat also handles docstring conventions and unused imports, but doesn't canonicalise them and we have linters for both.

IMO we should use the default configuration and accept 88-character lines. To minimise disruption, I would vote to format everything at once as soon as my current round of PRs is reviewed and merged.