chore(tox): add black and black-run commands

Laurent Peuch requested to merge topic/default/add-black-command into branch/default

Hello dear contributors,

This is an auto generated modification to help you improve the code base of logilab. If this commit is bad don't hesitate to close this MR and report it here https://forge.extranet.logilab.fr/cubicweb/cube-doctor/-/issues.

Kind regards,

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