Introduce event_loop_policy fixture

[seifertm]

This PR introduces the new fixture event_loop_policy. The fixture returns an instance of an asyncio.AbstractEventLoopPolicy. By default, the fixture has session (i.e. it is accessible by all tests) and returns the value of asyncio.get_event_loop_policy().

Users can run pytest-asyncio tests with different types of event loops by redefining the fixture to return a different event loop policy. Parametrizing the fixture causes pytest-asyncio tests to be run once for each specified policy. This allows writing tests against multiple different types of event loops.

Although the concept of event loop policies is likely going to be deprecated in CPython 3.13, it was decided that this functionality is still going to land in pytest-asyncio. Starting with CPython 3.13, pytest-asyncio will provide a migration path for the deprecated policy system.

Resolves #591
Prerequisite for #657

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (973eaa4) 94.08% compared to head (9c2b996) 94.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #662      +/-   ##
==========================================
+ Coverage   94.08%   94.31%   +0.22%     
==========================================
  Files           2        2              
  Lines         406      422      +16     
  Branches       87       89       +2     
==========================================
+ Hits          382      398      +16     
  Misses         14       14              
  Partials       10       10              

Files	Coverage Δ
pytest_asyncio/plugin.py	94.27% <100.00%> (+0.22%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Tinche]

Since I'm active on DPO, I just wanted to make sure you're aware of tentative plans to deprecate event loop policies in general (https://discuss.python.org/t/removing-the-asyncio-policy-system-asyncio-set-event-loop-policy-in-python-3-15/37553). From what I can tell, nothing has been decided yet, just bringing this to your attention.

Although I think the idea here is sound. If event loop policies are deprecated, we could allow users to override a different fixture (event_loop_configuration?) to apply simple customizations to event loops without overriding the event_loop fixture itself.

[seifertm]

Thanks for mentioning the policy deprecation!

The topic was also touched in #657 (comment) ff.
My takeaway from this thread was that we don't need to deprecate things faster than upstream.

If you also think the approach is generally sound, I'll continue to pursue this.