I have created a new GitHub bot called Bedevere ( https://github.com/python/bedevere; see the README if you don't know the Monty Python and the Holy Grail reference). Once activated on the cpython repo it will add a status check -- just like Travis and AppVeyor -- based on whether an issue number is found in a pull request's title. If it's there then the Details link will point to that issue on bugs.python.org (i.e. no more copying and pasting the number to get back to the issue). If it's missing then the status will be marking failing. For those PRs where an issue number is not necessary, you can add the "trivial" label and that will make the status be passing.
Long-term. Bedevere will probably gain other checks like whether there is a news entry (that solution is being actively worked on and I'm still hopeful will be in place this month). My hope is by having a single bot for these kinds of checks we can make it easier to automate what we can that doesn't make sense to do in the CI test builds (i.e. everything related to the PR itself, but not the code in the PR).
Now Bedevere has not been turned on yet because I'm sick and I don't want to try and diagnose issues the way I'm feeling, but I just wanted to let people know that the problem of people forgetting issue numbers is getting a fix (hopefully) shortly.