[lint] replace autopep8 with black (test/functional/interface_*)

Summary:
See D13876 for the description.

Depends on D13880

Test Plan:

arc lint -- test/functional/[a-mt]*.py
test/functional/test_runner.py interface_*

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13881