Improve code readability

⚠ Apply these changes on top of !1 (merged)!

Code readability improvements suggested by ruff and flake8.

ruff check src
flake8 --ignore=E231,E225,E226,E266,E262,E501,E265,F401,F403,E722 --max-line-length=96 src

(Note that the above tells flake8 to ignore many kinds of formatting errors...)