I haven't learned git yet so maybe my assumption that the original PR is on a branch/repo clone to which you can submit a PR is incorrect?
Bob requests that Alice merge bob/bob-feature into alice/master.
Alice requests that Bob merge alice/fix-bob-to-conform-to-pep8 into bob/bob-feature.
Bob merges that into bob/bob-feature, and Bob's PR into Alice's repo is now auto-updated to reflect the changes that he merged into his branch.
Alice is now satisfied, and merges bob/bob-feature with alice/master.
I haven't learned git yet so maybe my assumption that the original PR is on a branch/repo clone to which you can submit a PR is incorrect?