Fix unclosed event loop

[tjkuson]

pytest-asyncio temporarily switches the event loop policy whilst setting up managed tests and fixtures. Before this change, that policy switch also called asyncio.get_event_loop() to save the current loop and restore it later.

asyncio.get_event_loop() can create a loop when none is set, meaning pytest-asyncio recorded the newly created loop as if it were a pre-existing one, restored it after the runner finished, and did not close it. A later test calling asyncio.run() could clear the current loop, allowing that loop to be collected and reported as ResourceWarning: unclosed event loop.

This makes _temporary_event_loop_policy only swap and restore the policy. The scoped runner fixture still installs the runner loop explicitly for custom loop factories, where asyncio.Runner(loop_factory=...) does not do so itself, and clears it during teardown.

By returning asyncio.get_event_loop() to its original behaviour after an async test, it now means that any subsequent sync tests will see a closed event loop. I think this likely aligns with how users would expect pytest-asyncio to manage the event loop.

Closes #724

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.50%. Comparing base (af62924) to head (bbe8458).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1430      +/-   ##
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- Coverage   94.56%   94.50%   -0.06%     
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  Files           2        2              
  Lines         515      510       -5     
  Branches       62       62              
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- Hits          487      482       -5     
  Misses         22       22              
  Partials        6        6              

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[seifertm]

Thanks for the detailed explanation. We should absolutely get rid of any remaining use of get_event_loop .