Fix wrong event loop reference with mixed scoped tests

[seifertm]

Issue #950 describes a problem with tests that have mixed event loop scopes. When a function-scoped loop is executed in between two tests with wider loop scope, the tests after the function-scoped tests reference the wrong event loop.

This is caused by the implementation of the event_loop fixture and its finalizers. This PR addresses the issue twofold:

1. It changes the implementation of the event_loop fixture so that it properly restores the current event loop policy and event loop when the fixture is torn down.
2. It changes the event_loop fixture finalizers to only close loops that were not created by pytest-asyncio. This prevents the finalizers from accidentally closing a module-scoped loop provided by the plugin, for example.

Fixes #950

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.10%. Comparing base (581bd29) to head (7600594).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	90.32%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1029      +/-   ##
==========================================
- Coverage   91.42%   91.10%   -0.33%     
==========================================
  Files           2        2              
  Lines         548      562      +14     
  Branches       72       74       +2     
==========================================
+ Hits          501      512      +11     
- Misses         28       30       +2     
- Partials       19       20       +1     

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[asvetlov]

Looks good.
The code is extremely complex now; I have no idea how to simplify it and keep backward compatibility, though.

[seifertm]

Thanks for having a look!

I agree that the code is really complex. It takes hours to debug even simple changes and understand

We really made an effort to keep backwards compatibility, so that made the situation worse. Maybe we should start following through with all the deprecations that were introduced and start cleaning up the code.