[PR #6340/f6334e2 backport][3.9] Add an argument to override request body size (#6340)

[anesabml]

(cherry picked from commit f6334e2)

What do these changes do?

Are there changes in behavior for the user?

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."

[anesabml]

If this PR #6343 is closed because it's a new feature, should I close this too?

[asvetlov]

Please wait.
aiohttp master is for 4.0, it still has incomplete work that must be finished before 4.0 final.
If we will have enough new features in 3.9 and 4.0 is still blocked -- a new release will be 3.9.0 from 3.9 branch.
In 3.9 we cannot make backward-incompatible changes but still can add new features.

I know, I should fix 3.9 before merging. The branch is broken most likely after the previous backport.