Improve CI

[AlexWaygood]

Neither of these should be a massive priority, but there are two changes that might be good:

• Run mypy with --strict. There are still a fair few dynamically typed functions in the code base; these need type annotations to be added for us to start doing this.
• Add a pre-commit bot to auto-reformat a PR with black and isort. I think this dramatically improved the contributor experience over at typeshed. We obviously have far fewer contributors over at flake8-pyi, so it really doesn't need to be prioritised. However, this kind of auto-reformatting also makes it much easier to prepare (or start preparing) a PR on an iPad or mobile device, which is something I like to do :)

[JelleZijlstra]

I gave pre-commit.ci access to this repo. We'll need to add a pre-commit config file to get autoformatting like we do on typeshed.

[AlexWaygood]

We now run mypy with --strict in CI, and have made a number of other CI improvements, so I'm closing this.

Ideally we'd have a fully typed codebase, which would mean we wouldn't have to run mypy with --allow-subclassing-any,
--allow-untyped-defs and
--allow-incomplete-defs. But getting there depends on #183, so I don't think this issue needs to stay open anymore.