Using newer-style practices for strings

Hi,

This is my first contribution to GNOME in general. I'm interested in helping with refactoring and solving bugs in this project.

To the point of this MR, I wanted to start by using f-strings for string interpolation. It would make Python code easier to read and maintain. It might even have a tiny performance improvement regarding to its formatting approach.

Also for code that involved string concatenation, I used str.join() since concatenation is known to be slow and a discouraged practice.

I haven't finished going through all the Python modules in the lollypop directory, since I wanted to get feedback first.