Convert strings to f-strings
Since we drop Python 3.5 support (#240 (closed)), we can use the
f-string format everywhere.
What is f-string
#403 (closed): A part has been done by
str + strformats to
Try to keep the f-string readable by not putting a too big operation in the f-string.
For logging, keep
% format for now, since it causes issues with Pylint and mypy.