failing distributed tests on upstream-dev

[keewis]

Since dask/distributed#3706 gen_cluster requires functions decorated with it to be coroutines. We use it twice in test_distributed.py: on test_async and on test_serializable_locks, which both are pre-3.5 style coroutines (yield / yield from instead of async / await) but not marked as such (I'm not sure what pytest does with coroutines).

I think this should be a pretty strait-forward fix: replace def with async def and yield / yield from with await.