Use f-strings for string formatting

[EwoutH]

Reformats all the text from the old "%-formatted" and .format() formats to the newer f-string format, as defined in PEP 498. This requires Python 3.6+.

Flynt 0.76 was used to reformat the strings. 6 f-strings were created in 3 files.

F-strings are in general more readable, concise and performant. See also: https://www.python.org/dev/peps/pep-0498/#rationale