Switch to using the black formatter

[mpharrigan]

• copy check/format-incremental from Cirq (one minor edit to remove an unnecessary exclude line)
• copy black version pin from Cirq so our open-source dev environments are mildly in sync.

Instead of formatting everything all at once, I plan on just letting the formatting change slowly over time as PRs format only their changed files. cc @ncrubin @fdmalone @tanujkhattar

[fdmalone]

Why not format everything in one go @mpharrigan? It seems like formatting incrementally will increase noise during review.

[fdmalone]

cirq did this all in one go right, with some git-blame-ignore-revs?

[tanujkhattar]

cirq did this all in one go right, with some git-blame-ignore-revs?

Yes

[fdmalone]

With that being said, I'd rather switch to black in one way or another. Can follow up with a bulk reformat.

[fdmalone]

xref #835