Change __aiter__ return type to AsyncIterator

[blazewicz]

What do these changes do?

Change return type of MultipartReader.__aiter__() and BodyPartReader.__aiter__() to AsyncIterator.

Are there changes in behavior for the user?

No more mypy errors should for code using multipart.

Related issue number

It fixes #5163

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 #5165 into master will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5165      +/-   ##
==========================================
- Coverage   97.56%   97.45%   -0.11%     
==========================================
  Files          43       43              
  Lines        8780     8779       -1     
  Branches     1412     1412              
==========================================
- Hits         8566     8556      -10     
- Misses        100      112      +12     
+ Partials      114      111       -3     

Flag	Coverage Δ
#unit	97.45% <100.00%> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
aiohttp/multipart.py	96.27% <100.00%> (ø)
aiohttp/frozenlist.py	95.83% <0.00%> (-2.09%)	⬇️
aiohttp/http_writer.py	97.36% <0.00%> (-1.76%)	⬇️
aiohttp/http_parser.py	95.81% <0.00%> (-0.89%)	⬇️
aiohttp/http_websocket.py	98.37% <0.00%> (-0.28%)	⬇️
aiohttp/helpers.py	96.45% <0.00%> (-0.24%)	⬇️
aiohttp/hdrs.py	100.00% <0.00%> (ø)

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[asvetlov]

Thanks!

[github-actions]

💔 Backport was not successful

The PR was attempted backported to the following branches:

• ❌ 3.7: Commit could not be cherrypicked due to conflicts
• ❌ 3.8: Commit could not be cherrypicked due to conflicts

[asvetlov]

Backported manually