Fix some nits in bip68-112-113-p2p.py

Summary:

• Remove BytesIO dependency
• Refactor comments layout
• Fix test numbering

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix, schancel

Reviewed By: #bitcoin_abc, deadalnix, schancel

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2146