Compatibility with pytest 8

[frenzymadness]

According to: https://docs.pytest.org/en/latest/deprecations.html#id14 pytest 8 no longer supports setup/teardown functions/methods which leads to many failures like this:

____________________________ test_asyncio_interrupt ____________________________

    @pytest.mark.skipif(tornado.version_info < (5,), reason="only relevant on tornado 5")
    def test_asyncio_interrupt():
>       assert KM is not None
E       assert None is not None

ipykernel/tests/test_eventloop.py:67: AssertionError

Because these functions are ignored: https://github.com/ipython/ipykernel/blob/5534fd99b7c0a936392b35ad20bee026790332bd/tests/test_eventloop.py#L45-L56

[blink1073]

Thanks! I opened #1231.