> My reading of README.md is that Black inserts spaces around the colon, is that right?

I can't speak for the readme, but:

    $ cat test.py
    x = [1,2,3]
    print(x[1: 3])

    $ black test.py
    reformatted test.py

    $ cat test.py
    x = [1, 2, 3]
    print(x[1:3])