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.

The script has mostly been copied from alphaplan_fwr.py, but with
different input data.

Adding a job to the CI-test environment for testing this.