Use Python format strings instead of "{}" % var construct

[pabloyoyoista]

This is a more comfortable and modern way to format strings. It is supported since Python 3.6 and it's a lot more comfortable to use. I also bet there are young Python programmers (like new people that might get added to our datascience team) that have never seen anything different, since Python 3.6 was released in 2016

Ref: https://docs.python.org/3/reference/lexical_analysis.html#f-strings

Depends #8