Merge pull request #22541 from lidizheng/aio-poller-shutdown-error

[Aio] Fix AttributeError during deallocation of the completion queue

src/python/grpcio/grpc/_cython/_cygrpc/aio/grpc_aio.pyx.pxi

@@ -105,7 +105,7 @@ cdef _actual_aio_shutdown():
         )
         future.add_done_callback(_grpc_shutdown_wrapper)
     elif _global_aio_state.engine is AsyncIOEngine.POLLER:
-        _global_aio_state.cq.shutdown()
+        (<PollerCompletionQueue>_global_aio_state.cq).shutdown()
         grpc_shutdown_blocking()
     else:
         raise ValueError('Unsupported engine type [%s]' % _global_aio_state.engine)