Use Ruff for Python linting and formatting

Ruff is a widely used, extremely fast re-implementation of a number of Python tools like flake8, isort and black.

As proposed in #298 (closed) I changed the pre-commit configuration to use Ruff instead of the combination of the 3 tools flake8, isort and black.

The main differences wrt the previous configuration are

• black is replaced by ruff format command, isort replacement is an autofixable lint check (ruff check --select I --fix filename.py)
  • users never see this as it's wrapped in the pre-commit configuration
• there are small differences between Ruff and isort+black
  • https://docs.astral.sh/ruff/faq/#how-does-ruffs-import-sorting-compare-to-isort
  • https://docs.astral.sh/ruff/formatter/black/

Somehow Ruff reported a number of type-comparison (E721) violations that were not reported by flake8/pycodestyle, so I fixed them (as well as a few more that were not reported), but we may want to reconsider it (see https://github.com/astral-sh/ruff/issues/6465).

Closes #298 (closed)