There have been suggestions to use Black to format the mypy (and mypyc) codebase for a long time. Maybe it's time to actually do the switch now?

We'd need a volunteer to drive the project. I don't expect it to be a lot of work, but we'd need to at least update contributor documentation and pip dependencies. We'd also need to decide whether we'd fail a PR if it's not correctly formatted.

Next steps:

• Find a volunteer who'd like to drive this
• Collect feedback from contributors
• Write a more detailed plan about what needs to be done (if most contributors are happy with this or at least don't hate the idea)
• Merge some PRs as many of them will get merge conflicts after the switch? Or leave comments asking the contributors to run black themselves?
• Implement the switch