Replace flake8 with ruff

Ruff is much more modern and faster. Mainly, it does not slow down our pre-commit hooks that right now take ages due to flake8.

Particularly handy is that with ruff we can run `ruff check --fix netket` to fix most issues he identifies, speeding up development.

Moreover, it's settings are all stored in pyproject.toml so we can remove another file.