Backward compatibility for Python 3.5

[scottclowe]

Changes the format strings from f-strings (introduced in Python 3.6) to str.format() (which has been around since Python 2.6).

f-strings are great, but not universally supported yet - I made this change for myself because I'm using Ubuntu 16.04 LTS, which ships with Python 3.5.

I also needed to explicitly turn lists of objects into strings when they were being concatenated, using map(str, x).

Thought I'd share this in case you wanted to incorporate it into the main code base.