* Add pre-commit hook for black formatting

* Add black to requirements for dev

* Add black check to CI

* Update README with black logo and merge v2.1 changes

* Black reformat

* Fix black check path

* Reformat unit tests with black

* Check code style of unit tests with black on Travis

* Add section on Test_MPI to CONTRIBUTING.md

* Check code style for examples and benchmarks

* Update setup.py

Co-authored-by: Damian Hofmann <damian.hofmann@mpsd.mpg.de>