firmware: add CI script to run ruff check, and add a pyproject.toml

And fix some issues reported by ruff.

Also add a pyproject.toml file with some project information and ruff configuration. The pyproject.toml is based on the pmbootstrap one. I added Martin and "postmarketOS Developers" as authors, but that can very much be changed, please propose what you all would like there.

I looked at adding a CI job for pytest as well, but will hold off on that until tests are passing again :)