`_eventloop_set` event called from non-async context

The `_eventloop_set` AnyIO event cannot be set [here](https://github.com/ipython/ipykernel/blob/8cc1ee3b71de8273ca0fba4a560406adc05fdfde/ipykernel/kernelbase.py#L558), 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.