Add new tooling

[tumidi]

• remove tox (lives in qpy-dev now)
• add ruff as replacement for pylint/flake8
• fix tests, so they don't depend on CWD
• reformat codebase
• fix linter errors
• support for Python ~3.9, 3.10~ 3.11, 3.12
• comply to questionpy-org/questionpy-docs/pull/5
• use updated GitHub workflow
• update documentation (which seemed the right the thing to do while fixing D linter errors)