Convert strings to f-strings

Since we drop Python 3.5 support (#240 (closed)), we can use the f-string format everywhere.

What is f-string

• https://www.blog.pythonlibrary.org/2018/03/13/python-3-an-intro-to-f-strings/
• https://cito.github.io/blog/f-strings/

Todo

• #403 (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.