Event loop integration

[davidbrochart]

This issue is mostly for me to understand how we can move forward with the switch to AnyIO with regards to event loop integration.
It looks like this is currently done manually by the host event loop, which must call kernel.do_one_iteration() regularly.
I am wondering if we could use something like Trio's guest mode instead. There would be no need to worry about low-level details and manually run ipykernel's event loop.

cc @Carreau