Add missing pytest-asyncio to test extras
While testing for #48360 locally I got some errors due to `pytest-asyncio` missing after I did:

```
pip install -e .[test]
```

This should fix it!

I'm not sure how to begin debugging the test failure :) Any tips? @jorisvandenbossche, hoi! Sorry for the direct ping, I'm not sure who I should bother about this specific issue, and I'm not familiar with the typical PR cycle in pandas :) I'd like to not let this slip through the cracks: do you have pointers on how to solve the failures? Same goes for #48360. Thanks for the pull request. It looks like this was incorporated in https://github.com/pandas-dev/pandas/pull/47336, so going to close in favor of that pull request. But thanks for the contribution! 