Add code formatting (CI or pre-commit)

[gideongrinberg]

As discussed in #286 we might want to format code automatically. This could complement the linting that happens in our CI builds. Our two options would be adding formatting to the CI (not sure if that works, I know linting does) or using pre-commit. Pre-commit allows for linting and formatting before pushing. This enforces clean code, makes devs think about best practices and improves code review. This would be in addition to, not instead of, the CI checks.

Pre-Commit In Action

Demo Repo

Failed Hook

Sucessful Hook

[gideongrinberg]

See also #524 which discusses a style guide. If we want, we can set up either Flake8 or a formatter to follow this guide. If we have best practices for writing PPB programs (using PPB, not for PPB) we can also build a plugin for that.