Support addition of routes when handler is wrapped by functools.partial()

[yuvalof]

What do these changes do?

In aiohttp v4 a new check was added to make sure that route handlers are coroutines.
Python < v3.8 will return False for asyncio.iscoroutine(functools.partial(async_function)).
The change will iterate the wrapped function to make sure its async

Are there changes in behavior for the user?

No

Related issue number

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 #4302 into master will decrease coverage by 0.03%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #4302      +/-   ##
==========================================
- Coverage   97.58%   97.55%   -0.04%     
==========================================
  Files          43       43              
  Lines        8857     8862       +5     
  Branches     1387     1389       +2     
==========================================
+ Hits         8643     8645       +2     
- Misses         91       93       +2     
- Partials      123      124       +1

Impacted Files	Coverage Δ
aiohttp/web_urldispatcher.py	99.09% <100%> (-0.01%)	⬇️
aiohttp/helpers.py	96.15% <66.66%> (-0.44%)	⬇️
aiohttp/web_fileresponse.py	97.72% <0%> (-0.57%)	⬇️

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[asvetlov]

pls fix mypy: https://dev.azure.com/aio-libs/aiohttp/_build/results?buildId=1311&view=logs&j=cc4e4887-e527-5c83-25ce-76663593f2e1&t=9d25c5b1-1b8c-5097-b25c-a7f376352755&l=215

[yuvalof]

Fixed the pypy and merged master

[asvetlov]

thanks!

[nicktimko]

Also fixes #3252

[asvetlov]

Thanks @nicktimko