Use black for source code formatting

[rytilahti]

It makes sense to use black for code formatting as done many other projects recently. This commits mass-converts the code-base to use black and adds a precommit hook for future contributions.

• Black - "the uncompromising Python code formatter": /~https://github.com/psf/black

[coveralls]

Coverage decreased (-0.01%) to 72.223% when pulling b8989bb on blackify into d4354e3 on master.

[syssi]

I like the commit hook.