Remove deprecated event_loop_policy fixture

[seifertm]

The asyncio policy system has been deprecated and is scheduled for removal in CPython 3.16. The pytest-asyncio policy system is deprecated and has been superseeded by the pytest_asyncio_loop_factory hook.

This PR removes the deprecated event_loop_policy fixture.

Obsoletes #796

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.42%. Comparing base (054a52d) to head (2ac8576).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1517      +/-   ##
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- Coverage   94.50%   94.42%   -0.09%     
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  Files           2        2              
  Lines         510      502       -8     
  Branches       62       61       -1     
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- Hits          482      474       -8     
  Misses         22       22              
  Partials        6        6              

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[edgarrmondragon]

Should this context manager be outright removed? It still calls deprecated/removed APIs and all tests under Python 3.14 and 3.16 pass when I don't use it in _create_scoped_runner_fixture.

[tjkuson]

We discussed this offline at the pytest sprint. I think there are merits to either approach.

The idea was that code under test might apply an event loop policy which will need to be restored between tests even if we remove support for configuring the policy via the pytest-asyncio fixture. If we remove policy restoration context manager, users who depend on event loop policy being reset between tests could see confusing errors when upgrading to this new version (independent of the deprecated loop policy fixture being removed).

However, getting a warning about the deprecated API even after we ostensibly remove support for it is also a bad user experience. It might be strong enough reason to remove the support for event loop policies entirely.

If we remove both the fixture and the feature of the plugin that reset the policy between tests, then we'd need to communicate to users that both the fixture and support for testing code that sets an event loop policy are unsupported in v2.

[edgarrmondragon]

Thanks for the context! So perhaps there is a compromise where the current policy is guarded and restored only on Python < 3.16, since users won't presumably be able to apply any policies on 3.16+ anyway. But perhaps that's thinking way too ahead of time.