Reformat bip68-112-113-p2p.py and apply various nits

Summary: As per title.

Test Plan:

./test/functional/test_runner.py bip68-112-113-p2p.py

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1124