use f-strings to simplify the code

[totaam]

I had not realized that f-strings were included in Python 3.6

This can make the code more readable (started in 4c3f0af)

Only to be used in places where the extra cost of evaluating the logging statement won't affect performance.
(some debug statements rarely get printed and should continue to only be evaluated when they are actually used)

[totaam]

This will do for now.

One small downside is that it can be harder to grep before {VARNAME} after than before %s after.