Upgrade ruff to 0.9.1

[DimitriPapadopoulos]

• Fix RUF100 new warnings (E402 unused).
• Enable ISC001/ISC002 since the documentation now states that they are compatible with the ruff formatter if used together:

  When using Ruff as a formatter, we recommend avoiding the following lint rules:
  [...]
  multi-line-implicit-string-concatenation (ISC002) if used without ISC001 and flake8-implicit-str-concat.allow-multiline = false

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)