chore: use AnyIO 4.0+ in tests dependencies

chore: use AnyIO 4.0+ in tests dependencies

We set the 'loop_factory' asyncio option for parametrizing the
anyio_backend() fixture as anyio 4.0 has replaced the 'policy' option by
'loop_factory'.

In tests/pool/test_pool_async_noasyncio.py, we previously used
the anyio_backend_options fixture to retrieve the loop policy; now we
build it directly. This could be improved by using the loop_factory, but
only from Python 3.11 with an asyncio.Runner().