Since PRs on Github auto-update when the branch they are based on is updated, they would not need to create a new pull request:

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.