[python-committers] Bedevere now automatically removes "needs backport to *" labels

When you create a backport PR, if the title is formatted as, e.g. "[3.6] stuff that changed (GH-1234)", then Bedevere will remove the "needs backport to 3.6" label on the GH-1234 PR and leave a comment linking to the backport PR that triggered the label removal.