`_eventloop_set` event called from non-async context

[davidbrochart]

The _eventloop_set AnyIO event cannot be set here, because it may be called from a non-async context. This event was introduced in https://github.com/ipython/ipykernel/pull/1265 and the issue was revealed in https://github.com/ipython/ipykernel/pull/1291 when running on a trio backend.