Add Trove classifier for Python 3.13

[hugovk]

dependencies = [
  "cffi>=v1.17.0rc1", # remove once v1.17.0 is out; add 3.13 classifier above (see #127)

cffi 1.17.1 is now out: https://pypi.org/project/cffi/

Also replace Flake8 with Ruff, and add some extra linting.