Scoped event loops based on pytest marks

[seifertm]

This PR introduces the asyncio_event_loop mark. When applied to classes and modules, the mark provides an asyncio event loop that is class-scoped or module-scoped, respectively. This is a prerequisite for the deprecation of event_loop fixture overrides (see #631).

Fixtures and tests under the same asyncio_event_loop mark are run in the same event loop, thus addressing #127 in parts.

The asyncio_event_loop mark supports an optional policy keyword argument, which takes an event loop policy or an iterable of policies. Tests under the effect of the mark are parametrized over the provided policies. If no policy is provided, the mark falls back to the value of asyncio.get_event_loop_policy() at mark definition time. This functionality offers a workaround for #591.

The implementation dynamically creates pytest fixtures based on the presence of the asycnio_event_loop mark. The fixtures are anchored at the corresponding module or class. The generated fixtures have a unique name that is generated from the pytest node ID of the module or class.

The generated fixtures are always parametrized, even when just a single event loop policy is specified. Therefore, we need to make pytest aware which parameters to use when evaluating the fixture. This happens in pytest_generate_tests. Whenever pytest_generate_tests is called for a test that is affected by an asyncio_event_loop mark_, the name of the dynamic fixture is injected into the metafunc along with the dynamic fixture definition. This information allows pytest to perform the standard fixture parametrization.

There is one special case, where a user may have requested the event_loop fixture for an async test that also requests a scoped event loop. In this case, the code raises an error, because we cannot possibly know what the user intended to do here.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (c99ef93) 93.95% compared to head (9392b20) 93.49%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
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- Coverage   93.95%   93.49%   -0.46%     
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  Files           2        2              
  Lines         281      338      +57     
  Branches       55       70      +15     
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+ Hits          264      316      +52     
- Misses         11       12       +1     
- Partials        6       10       +4     

Files	Coverage Δ
pytest_asyncio/plugin.py	93.43% <93.42%> (-0.46%)	⬇️

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[seifertm]

Added support for module-scoped loops.

[seifertm]

Added support for specifying a single event loop policy as a keyword argument to the asyncio_event_loop mark.

[seifertm]

• Added support for multiple policies to be specified via the asyncio_event_loop mark. Tests under the same mark are parametrized accordingly.
• Deprecated overrides of the event_loop fixture in favour of the asyncio_event_loop mark.

[seifertm]

I decided to move the deprecation of the event loop fixture into a separate PR.
This PR is a feature PR only.