Port the abcbot code into this repository

Summary:
The abcbot (more appropriately called buildbot) code lived outside of
this repo for a while. It belongs here so that we can:

• Increase visibility with other developers
• More atomically make changes to the bot and other software and/or tests
• Reduce bottlenecks that are currently dependent on Fabien and I
• Integrate its tests with our existing CI
• Get linting practically for free

This patch is a linted version, so will differ from the original.

I copied the files directly in order to remove some sensitive git history.
Should you come across this diff in a git blame, please also consider
reaching out to Fabien, as he is also a current co-author/maintainer.

Depends on D7326

Co-authored-by: Shammah Chancellor <shammah.chancellor@gmail.com>
Co-authored-by: Fabien <fabcien@gmail.com>

Test Plan:

cd buildbot/test
workon abcbot   # see readme for details
pytest

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7328