Enforce string-formatting with f-strings
# What does this PR do?

This PR removes any strings formatted with `.format` or `%` to use f-strings exclusively (unless there is a very good reason to use the other syntax, or the file is in a research_project/legacy folder).

The mix of three syntaxes does not make any sense and we all agree in the team that f-strings are more readable. Now that Python 3.5 is officially dead, there is no reason not to switch fully to f-strings.

cc @stas00  as we had a conversation about that.
