Replace event loops with asyncio.Runner

[seifertm]

Previously, pytest-asyncio used a combination of low-level asyncio functions to manage event loops. This patch replaces the low-level functions with the use of asyncio.Runner.

The runner takes care of cancelling remaining tasks at the end of its scope, avoiding errors about pending tasks. This also fixes an issue that caused RuntimeErrors in abanoned tasks that called functions which depend on a running loop.

Moreover, the use of asyncio.Runner and its backport allows backporting the propagation of contextvars from fixtures to tests to Python 3.9 and Python 3.10.

Supersedes #309
Closes #127
Closes #200
Closes #878
Partially addresses #1032

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.48649% with 5 lines in your changes missing coverage. Please review.

Project coverage is 89.01%. Comparing base (6c9914b) to head (ce15320).

Files with missing lines	Patch %	Lines
pytest_asyncio/plugin.py	86.48%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1152      +/-   ##
==========================================
- Coverage   89.40%   89.01%   -0.39%     
==========================================
  Files           2        2              
  Lines         434      428       -6     
  Branches       51       51              
==========================================
- Hits          388      381       -7     
- Misses         31       33       +2     
+ Partials       15       14       -1     

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