Enforce formatting style?

[mesca]

Formatting style across the project is inconsistent.
I would suggest to fix this issue by using Black. It works pretty well, and it is very easy to use.

[agramfort]

I am a bit reluctant to do such a massive reformatting of a code. It breaks PRs and you loose the origin of the code (git blame harder). I think there are more urgent things to do. Migrate to GH actions, add flake8 to CI, move from nose to pytest etc.

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