[PR #8283/54e13b0a backport][3.9] Fix blocking I/O in the event loop while processing files in a post request

[patchback]

This is a backport of PR #8283 as merged into master (54e13b0).

What do these changes do?

Fix blocking I/O in the event loop while processing files in a post request

It looks like this was introduced in f2dbdf9

Are there changes in behavior for the user?

event loop is not blocked

Is it a substantial burden for the maintainers to support this?

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
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  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES/ folder

  • name it <issue_or_pr_num>.<type>.rst (e.g. 588.bugfix.rst)

  • if you don't have an issue number, change it to the pull request
    number after creating the PR

    • .bugfix: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
      pre-agreed expectations.
    • .feature: A new behavior, public APIs. That sort of stuff.
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      changes in behavior.
    • .breaking: When something public is removed in a breaking way.
      Could be deprecated in an earlier release.
    • .doc: Notable updates to the documentation structure or build
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    • .contrib: Stuff that affects the contributor experience. e.g.
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    • .misc: Changes that are hard to assign to any of the above
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  • Make sure to use full sentences with correct case and punctuation,
    for example:

    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.

    Use the past tense or the present tense a non-imperative mood,
    referring to what's changed compared to the last released version
    of this project.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.42%. Comparing base (3f79241) to head (8f7a89e).

Additional details and impacted files

@@           Coverage Diff           @@
##              3.9    #8293   +/-   ##
=======================================
  Coverage   97.42%   97.42%           
=======================================
  Files         108      108           
  Lines       32797    32800    +3     
  Branches     3876     3876           
=======================================
+ Hits        31953    31956    +3     
  Misses        640      640           
  Partials      204      204           

Flag	Coverage Δ
CI-GHA	97.33% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.01% <100.00%> (+<0.01%)	⬆️
OS-Windows	94.61% <100.00%> (+<0.01%)	⬆️
OS-macOS	96.73% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	94.43% <100.00%> (+<0.01%)	⬆️
Py-3.10.14	96.79% <100.00%> (-0.02%)	⬇️
Py-3.11.8	97.05% <100.00%> (+<0.01%)	⬆️
Py-3.12.2	97.09% <100.00%> (-0.01%)	⬇️
Py-3.8.10	94.41% <100.00%> (+<0.01%)	⬆️
Py-3.8.18	96.73% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	94.41% <100.00%> (+<0.01%)	⬆️
Py-3.9.19	96.78% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.15	96.32% <100.00%> (+<0.01%)	⬆️
VM-macos	96.73% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.01% <100.00%> (+<0.01%)	⬆️
VM-windows	94.61% <100.00%> (+<0.01%)	⬆️

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