Convert strings to f-string format as much as possible

Since we drop Python 3.5 support, we can use the f-string format everywhere.

• #186 (closed): A part has been done by pyupgrade.
• Convert manually .format(), %, and str + str formats to f-string format.

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.