Add cdef type declarations and inline upgrade check in HTTP parser

[bdraco]

What do these changes do?

Add cdef type declarations for h_upg and enc local variables in _on_headers_complete, and inline the allowed variable into the conditional check. Noticed while reviewing #11966

Are there changes in behavior for the user?

No behavior changes; this is a minor optimization to help Cython generate better C code.

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

No, this is a small change with no maintenance burden.

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
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• Add a new news fragment into the CHANGES/ folder

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  • 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
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    • .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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    • .misc: Changes that are hard to assign to any of the above
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    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,
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.11%. Comparing base (d9bd1be) to head (dc99fd7).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12321   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files         130      130           
  Lines       45558    45558           
  Branches     2404     2404           
=======================================
+ Hits        45155    45156    +1     
  Misses        272      272           
+ Partials      131      130    -1     

Flag	Coverage Δ
CI-GHA	98.97% <ø> (+<0.01%)	⬆️
OS-Linux	98.71% <ø> (ø)
OS-Windows	96.99% <ø> (ø)
OS-macOS	97.87% <ø> (+<0.01%)	⬆️
Py-3.10.11	97.42% <ø> (+<0.01%)	⬆️
Py-3.10.20	97.89% <ø> (ø)
Py-3.11.15	98.09% <ø> (-0.01%)	⬇️
Py-3.11.9	97.63% <ø> (+<0.01%)	⬆️
Py-3.12.10	97.71% <ø> (-0.01%)	⬇️
Py-3.12.13	98.19% <ø> (ø)
Py-3.13.12	98.44% <ø> (-0.01%)	⬇️
Py-3.14.3	98.49% <ø> (ø)
Py-3.14.3t	97.50% <ø> (-0.01%)	⬇️
Py-pypy3.11.15-7.3.21	97.38% <ø> (+<0.01%)	⬆️
VM-macos	97.87% <ø> (+<0.01%)	⬆️
VM-ubuntu	98.71% <ø> (ø)
VM-windows	96.99% <ø> (ø)

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[codspeed-hq]

Merging this PR will not alter performance

✅ 59 untouched benchmarks

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_{Comparing bdraco:cython-type-declarations-http-parser (dc99fd7) with master (e412ccb)}