> Thank you. I'll re-base my branch onto the master branch found at the GitHub repository.

You may re-clone a GitHub repository instead of rebase bitbucket repo.


> I don't maintain repositories at GitHub. Does that mean the contribution must be rejected?

No, your contribution will not be rejected by "you don't maintain the repo on GitHub", everyone can send the patch (PR) to python/cpython.

you will need to fork python/cpython from GitHub first, this will create a fork in your GitHub account, maybe benf/cpython if your GitHub account is benf, and then clone this repo (benf/cpython), the rest step please see the instruction in devguide !