revert changes to abc-p2p-compactblock.py

Summary: Except D2100, for which there isn't much of a problem.

Test Plan: The test still fails, but at least it'll allow to land the fix.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2103