[PR #8252/8f237126 backport][3.9] Fix handling of unsupported upgrades with the pure python http parser

[patchback]

This is a backport of PR #8252 as merged into master (8f23712).

What do these changes do?

Fix handling of unsupported upgrades with the pure python http parser

The content is now read from an unsupported upgrade request

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
  • 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_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.
    • .deprecation: A declaration of future API removals and breaking
      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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    • .packaging: Notes for downstreams about unobvious side effects
      and tooling. Changes in the test invocation considerations and
      runtime assumptions.
    • .contrib: Stuff that affects the contributor experience. e.g.
      Running tests, building the docs, setting up the development
      environment.
    • .misc: Changes that are hard to assign to any of the above
      categories.

  • 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 97.36%. Comparing base (c427e6d) to head (9879c32).
Report is 2 commits behind head on 3.9.

Additional details and impacted files

@@           Coverage Diff           @@
##              3.9    #8254   +/-   ##
=======================================
  Coverage   97.36%   97.36%           
=======================================
  Files         108      108           
  Lines       32637    32639    +2     
  Branches     3874     3874           
=======================================
+ Hits        31778    31780    +2     
  Misses        655      655           
  Partials      204      204           

Flag	Coverage Δ
CI-GHA	97.27% <100.00%> (+<0.01%)	⬆️
OS-Linux	96.96% <100.00%> (+<0.01%)	⬆️
OS-Windows	94.67% <100.00%> (+0.19%)	⬆️
OS-macOS	96.77% <100.00%> (-0.01%)	⬇️
Py-3.10.11	94.37% <100.00%> (-0.01%)	⬇️
Py-3.10.13	96.49% <100.00%> (-0.26%)	⬇️
Py-3.10.14	96.71% <100.00%> (?)
Py-3.11.8	96.79% <100.00%> (+0.28%)	⬆️
Py-3.12.2	96.84% <100.00%> (?)
Py-3.8.10	94.36% <100.00%> (+<0.01%)	⬆️
Py-3.8.18	96.68% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	94.37% <100.00%> (+<0.01%)	⬆️
Py-3.9.18	96.47% <100.00%> (-0.26%)	⬇️
Py-3.9.19	96.69% <100.00%> (?)
Py-pypy7.3.15	96.27% <100.00%> (+<0.01%)	⬆️
VM-macos	96.77% <100.00%> (-0.01%)	⬇️
VM-ubuntu	96.96% <100.00%> (+<0.01%)	⬆️
VM-windows	94.67% <100.00%> (+0.19%)	⬆️

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