Switch all strings to formatted string literals (f-strings)

The old printf-style string formatting syntax using '%' is not recommended anymore. See for reference: https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting https://docs.sourcery.ai/Reference/Python/Default-Rules/replace-interpolation-with-fstring/

Python 3.6 introduced f-strings as a better alternative: https://docs.python.org/3/reference/lexical_analysis.html#f-strings

Consequently switch to this new syntax for all strings.