[3.6] switch noop from function to class (#4322)

[Jackenmen]

What do these changes do?

They remove the RuntimeWarning for noop function by turning it into class with __await__. It's a backport of #4322 to aiohttp 3.6

Are there changes in behavior for the user?

No

Related issue number

#4282

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-io]

Codecov Report

Merging #4712 into 3.6 will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              3.6    #4712      +/-   ##
==========================================
- Coverage   97.96%   97.92%   -0.04%     
==========================================
  Files          44       44              
  Lines        8886     8886              
  Branches     1392     1392              
==========================================
- Hits         8705     8702       -3     
- Misses         72       75       +3     
  Partials      109      109              

Impacted Files	Coverage Δ
aiohttp/helpers.py	96.51% <100.00%> (ø)
aiohttp/web_protocol.py	91.34% <0.00%> (-0.97%)	⬇️

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