[lint] replace autopep8 with black (test/functional/rpc_*)
Summary:
See D13876 for the description.
Depends on D13885
Test Plan:
arc lint -- test/functional/[a-v]*.py test/functional/test_runner.py rpc_*
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D13886