Historically commit messages for CPython have had the form of "Issue #NNNN: did something". The problem is that Github automatically links "#NNNN" to GitHub issues (which includes pull requests). To prevent incorrect linking we need to change how we reference issue numbers.
The current candidates are:
issue NNNN (notice the lack of #)
bug NNNN
Whatever choice we go with it will be how we reference issues in PR titles and comments to link the PR to the issue, and in commit messages to send a message to the issue about the commit.
To start this off, I'm -1 on "issue" (because people will out of habit add the #), +0 on "bug" (it's different but not everything is a bug), and +1 on "bpo" (as it namespaces our issues).