Fix usage of `asyncio.get_event_loop` which was changed in Python 3.12 to raise when no event loop was set

[sphuber]

This is circumvented when going through a policy, which is what plumpy does, but that also raises a deprecation warning and is expected to be removed in the future.

[danielhollas]

Link to docs:

https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.get_event_loop

Code that raises the warning:

plumpy/src/plumpy/events.py

Line 42 in 55e05e9

self._loop = super().get_event_loop()

[unkcpz]

I am trying to address it in #308. For our own defined policy I think we can, but the warning still raised from pytest.asyncio and nest_asyncio. I'll try to make the event loop usage more explicit and first try to get rid of the deprecated way we use get_event_loop.