[lint] replace autopep8 with black (contrib/buildbot)

Summary:
This concludes the sunsetting of autopep8 in favor of black, for a much more deterministic formatting (less dependant on the input format).

Depends on D13892 and D13893

Test Plan: invoke the buildbot

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13894