QAsyncioEventLoopPolicy always shuts down QCoreApplication

QAsyncioEventLoopPolicy always shuts down QCoreApplication when the event loop closes.

It should be possible to control this behaviour (toggle on/off).

An example where this is problematic is the pytest-qt library which completely breaks when each unit test creates an event loop instance, since pytest-qt intends to reuse a single QCoreApplication across the test suite, which is made impossible by this change in Pyside 6.6.1.