GitHub action (#1421)

* Implement a re-usable GitHub Action

Implement a GitHub action that can be reused across projects that want
to run black as part of their CI workflows.

* Fix typo in README.md

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

* Use latest Python 3

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>