Black isort

[psteinb]

Sorry to nag. While working on #19, I ran isort and black on this repo as suggested by CONTRIBUTING.md. This PR marks the result of first running:

$ black --version
black, version 21.10b0

and then

$ isort --version     

                 _                 _
                (_) ___  ___  _ __| |_
                | |/ _/ / _ \/ '__  _/
                | |\__ \/\_\/| |  | |_
                |_|\___/\___/\_/   \_/

      isort your imports, so you don't have to.

                    VERSION 5.10.0

from the toplevel folder of the repo. I also adopted the instructions on using isort.

[jan-matthis]

Thanks @psteinb -- I merged this via #25 to have the commits more incremental