Ensure async fixture setup and teardown run in the same task

[provinzkraut]

An attempt to fix #1191.

Inspired by anyio's pytest plugin: https://github.com/agronholm/anyio/blob/b8e91a5cfe35c3b28f91ea8251674a871a7f4e26/src/anyio/_backends/_asyncio.py#L2222-L2239.

Unfortunately, this requires some hacky hacks abusing backports.asyncio.runner to ensure task context is properly propagated on Python < 3.11, in order not to break things.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.18182% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.28%. Comparing base (3ab1fea) to head (7581198).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	93.18%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1193      +/-   ##
==========================================
+ Coverage   88.91%   89.28%   +0.36%     
==========================================
  Files           2        2              
  Lines         406      448      +42     
  Branches       44       48       +4     
==========================================
+ Hits          361      400      +39     
- Misses         35       37       +2     
- Partials       10       11       +1     

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[seifertm]

Thanks for sharing this! In general, I think we can try something like this.

It expect this to be a very intrusive change that has the potential to break a lot. Therefore, the queue-based task runner needs to be behind a feature toggle, such as a configuration value and/or a command line option.

I'm not sure what the best approach is codewise. My very early guess it that it would help to create some kind of abstraction on top of asyncio.Runner, and move some existing log, such as handling of contextvars, there. In the next step, we can create our new queue-based implementation and eventually we can use the feature toggle to switch between them. This is just an idea, though. I have no clue how it plays out.