As Karthikeyan said, you do need to open a PR against the CPython master.  I'm not a Git expert, but this page (section 3.3) discusses the steps to open that PR:
https://devguide.python.org/pullrequest/

Those are the steps I follow and it's a good write-up so there's not much I can add to it.  I don't think the PR on your own fork was required.  It doesn't usually ask to delete the branch until the PR against CPython master has been merged (which is the last step in section 3.3), but since you merged against your own fork, that's probably why it's asking now.  Do you still get an option to create a PR against CPython master using your branch as per this part of section 3.3?

> Finally go on https://github.com/<your-username>/cpython: you will see a box with the branch you just pushed and a green button that allows you to create a pull request against the official CPython repository.