Do post-merge checks on PRs to remind committers what to do when they forget something

[brettcannon]

E.g. rewrite the commit message, change #12345 to GH-12345, etc. The idea is that if after a merge it's discovered a core dev forgot to do something then a comment on the PR can be left reminding them what to try and remember what to do next time.

https://mail.python.org/pipermail/python-committers/2017-July/004678.html
https://mail.python.org/pipermail/python-committers/2017-August/004788.html

[brettcannon]

/cc @berkerpeksag @serhiy-storchaka

[Mariatta]

This is tracked in python/bedevere#14, so I'm closing this.
The only thing left is about reminding people to cleanup the asterisks in the commit message. (Number 3 in the issue).