bpo-36373: Deprecate explicit loop parameter in all public asyncio APIs [task] [WIP]

[eamanu]

This PR deprecate explicit loop parameters in all public asyncio APIs

This issues is split to be easier to review.

First step: tasks.py

https://bugs.python.org/issue36373

[tirkarthi]

Please handle DeprecationWarning in the tests. Running with -Werror would give test failures due to warnings.

[asvetlov]

Agree with @tirkarthi
./python -Werror -m test should pass
You can find the following command useful in development:
`./python -Werror -m test -v test_asyncio"

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

And if you don't make the requested changes, you will be poked with soft cushions!

[asvetlov]

Please run the PR with ./python -Werror -m test --check-env-changed -v test_asyncio and make sure that the test run is succeded.

[asvetlov]

This check is redundant: asyncio.gather() always passes loop to internal _GatheringFuture constructor.

[asvetlov]

Task is called with explicit loop by loop.create_task() method, please drop the check

[codecov]

Codecov Report

Merging #13670 into master will decrease coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master   #13670      +/-   ##
==========================================
- Coverage   82.78%   82.78%   -0.01%     
==========================================
  Files        1842     1842              
  Lines      559265   559276      +11     
  Branches    41382    41387       +5     
==========================================
- Hits       463009   462990      -19     
- Misses      87188    87209      +21     
- Partials     9068     9077       +9

Impacted Files	Coverage Δ
Lib/asyncio/tasks.py	94.73% <66.66%> (-0.77%)	⬇️
Lib/idlelib/runscript.py	19.09% <0%> (-1.82%)	⬇️
Lib/idlelib/scrolledlist.py	36.6% <0%> (-1.79%)	⬇️
Lib/test/test_asyncio/functional.py	70.65% <0%> (-1.64%)	⬇️
Lib/heapq.py	94.53% <0%> (-1.57%)	⬇️
Lib/idlelib/squeezer.py	91.94% <0%> (-1.35%)	⬇️
Lib/test/test_capi.py	93.4% <0%> (-0.8%)	⬇️
Lib/_weakrefset.py	59.18% <0%> (-0.69%)	⬇️
Lib/socketserver.py	80% <0%> (-0.34%)	⬇️
Lib/test/test_random.py	96.43% <0%> (-0.31%)	⬇️
... and 16 more

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[asvetlov]

The work is merged as a part of #16033