Reformat abandonconflict.py and backport PR10344
Reformat abandonconflict.py and backport PR10344

Summary: As per title.

Test Plan:
  ./test/functional/test_runner.py abandonconflict

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1151

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