Introduce Black to automate code formatting

Use Black, the uncompromising Python code formatter, to format the
project and simplify contributions. Contributors no longer need to think
or care about the project's preferred style, just let the tools handle
it instead.

Black is becoming more and more popular across Python projects and has
recently became a project under the Python Software Foundation.

This is especially important with the adoption of mypy as type
annotations create another obstacle or consideration in code formatting,
but Black handles this just fine.

Many projects use both Black and isort. Using Black internally also
helps ensure continued compatibility between the two popular projects.

For more details on Black, see:

https://github.com/psf/black

> Black is the uncompromising Python code formatter. By using it, you
> agree to cede control over minutiae of hand-formatting. In return,
> Black gives you speed, determinism, and freedom from pycodestyle
> nagging about formatting. You will save time and mental energy for
> more important matters.