The pull request you opened was invalid, because it was just trying to merge the `main` branch of the python/cpython repository into the `3.9` branch.  What we need instead is a pull request between a branch from `main` in your own fork of python/cpython with the change you want to see in a new commit.  You can find a more detailed walkthrough in the devguide, or if you prefer to leave it for someone else to do that is also a valid option :)