Adopt Knocean's new Python style guidelines

[jamesaoverton]

I'd like two sets of changes to the style of the code.

1. Use Python 3.6's new f-string syntax throughout. I think this is much easier to read.

2. Use flake8 and black to enforce a consistent style. I admit that it's annoying to make this change, but the benefit is that we never need to worry about style issues again.