Add configuration file for `black` defaults
Add configuration file for `black` defaults

The `black` project decided not to support `setup.cfg`, but only
`pyproject.toml` (see https://github.com/python/black/issues/65).

In order to avoid multiple configuration places for
our `black` defaults, I added this additional configuration file to
the repo.

hackaugusto/raiden,hackaugusto/raiden