Async: cancelling the task that holds the processing lock leaves queued send() callers pending #690

On the async engine, when the coroutine that is running the processing loop is cancelled (for example by asyncio.wait_for or a request timeout around await sm.send(...)), other coroutines awaiting send() of events queued behind it never return. CancelledError is a BaseException, so the exception path added in #688 does not run.

class N(StateMachine):
    a = State(initial=True); b = State()
    go = a.to(b); step = a.to.itself() | b.to.itself()
    async def on_go(self): await asyncio.sleep(10)
    def on_step(self): return "stepped"

holder = asyncio.create_task(sm.send("go"))
await asyncio.sleep(0.05)
other = asyncio.create_task(sm.send("step"))
await asyncio.sleep(0.05)
holder.cancel()
await asyncio.wait_for(other, 2)  # TimeoutError: never answered

A later send() works, so the machine recovers, but the queued callers stay pending forever. Reported as pre-existing on v3.2.1 in #688. Expected: queued callers are answered (their events processed, or rejected) instead of hanging.