Convert Python code from "%-formatted" and .format(...) strings into f-strings
Remove existing conversion script that only handles %-formatted strings.
TODO:
- wait for a new flynt release to have the stdin/stdout features
- check the next release version is indeed 0.78
- if the next release includes https://github.com/ikamensh/flynt/pull/164, then we don't need the stdin business: remove the overloading of buildfutures
- check if the next release fixes the extra newline added to stdout (I mentionned it to the devs on github)