Add a Scheduling Protocol

- Clearly delineate the required interface of schedulers
- Useful for typing

src/xdist/scheduler/__init__.py

@@ -3,4 +3,5 @@ from xdist.scheduler.load import LoadScheduling as LoadScheduling
 from xdist.scheduler.loadfile import LoadFileScheduling as LoadFileScheduling
 from xdist.scheduler.loadgroup import LoadGroupScheduling as LoadGroupScheduling
 from xdist.scheduler.loadscope import LoadScopeScheduling as LoadScopeScheduling
+from xdist.scheduler.protocol import Scheduling as Scheduling
 from xdist.scheduler.worksteal import WorkStealingScheduling as WorkStealingScheduling