Replace legacy string formatting with f-strings

[quantum5]

We can replace all legacy %-formatting and the majority of the slightly newer .format formatting with f-strings. This is a bit of a long term project and we don't expect it to be done anytime soon.

[Riolku]

I'll work on this: we can make use of https://pypi.org/project/flake8-use-fstring/ (https://github.com/MichaelKim0407/flake8-use-fstring). I'll see when I can get a PR up.

[quantum5]

You should probably get PyCharm then. It can autoconvert all sorts of string formatting, including the obscure options that we use, to f-strings.

[Riolku]

Alright, I might look into it. Alternatively... I might see if there's something for Atom.

[quantum5]

Here's an example:

pycharm.mp4

[Riolku]

Ooh, that is quite nice... maybe I'll have to install it if I don't find anything for atom...