[PR #5862/7080a8bb backport][3.8] Use MultiLoopChildWatcher in tests where available

[patchback]

This is a backport of PR #5862 as merged into master (7080a8b).

What do these changes do?

Python 3.8 adds a new child watcher class that doesn't require running loop in the main thread: https://docs.python.org/3/library/asyncio-policy.html#asyncio.MultiLoopChildWatcher. This PR uses the new watcher if it is available in asyncio module.

It should reduce test flakiness when parallelized with pytest-xdist.

Are there changes in behavior for the user?

No change for end users. Existing tests in test_loop.py covers this change given the current text matrix.

Related issue number

#3450
#2075

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov]

Codecov Report

Merging #5863 (7080a8b) into 3.8 (8ce3013) will decrease coverage by 0.78%.
The diff coverage is 96.31%.

❗ Current head 7080a8b differs from pull request most recent head 52d8823. Consider uploading reports for the commit 52d8823 to get more accurate results

@@            Coverage Diff             @@
##              3.8    #5863      +/-   ##
==========================================
- Coverage   97.53%   96.75%   -0.79%     
==========================================
  Files          44       44              
  Lines        8844     9851    +1007     
  Branches     1424     1591     +167     
==========================================
+ Hits         8626     9531     +905     
- Misses        102      182      +80     
- Partials      116      138      +22     

Flag	Coverage Δ
unit	96.65% <96.07%> (-0.73%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/web_middlewares.py	100.00% <ø> (ø)
aiohttp/web_protocol.py	86.41% <ø> (-4.20%)	⬇️
aiohttp/web_request.py	95.99% <ø> (-1.57%)	⬇️
aiohttp/web_response.py	98.21% <ø> (-0.06%)	⬇️
aiohttp/web_routedef.py	98.11% <ø> (+0.01%)	⬆️
aiohttp/web_runner.py	97.74% <ø> (+0.11%)	⬆️
aiohttp/web_server.py	94.28% <ø> (-5.72%)	⬇️
aiohttp/web_urldispatcher.py	97.83% <ø> (+0.23%)	⬆️
aiohttp/web_ws.py	91.53% <ø> (ø)
aiohttp/worker.py	94.91% <ø> (-1.81%)	⬇️
... and 75 more

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