Python 3.11: DeprecationWarning: There is no current event loop

When running the test suite on Python 3.11, tests with a `@pytest.mark.asyncio` decorator, trigger a DeprecationWarning:

```
.../python3.11/site-packages/pytest_asyncio/plugin.py:884: DeprecationWarning: There is no current event loop
   _loop = asyncio.get_event_loop()
```