Add black formatter and format files

[wamsiv]

fixes #251
fixes #284

[randyzwitch]

Does this PR just trade Black for Flake, or will it auto-format code using Black when someone submits a PR?

[wamsiv]

@randyzwitch Black format check will be added along Flake checks and will not auto format code just like Flake.

[randyzwitch]

I'm not sure I see the value of a gigantic diff, just to switch linters. At least for Atom, there are both flake8 and Black auto-formatting plugins on save, so what do we gain by moving to Black?

[randyzwitch]

Closing this for now, fixed conftest issue. Will revisit Black once it's clearer how we can do this automatically

[xmnlab]

Ibis has black inside the git pre-commit hook and it uses also the pre-commit as a mechanism for checking inside CI. So it helps to send code in the desired format and the CI will check if the author used pre-commit. ibis has also other services inside pre-commit such as flake8 and isort.

maybe it could be helpful for pymapd now or in the future