Improve performance of _serialize_headers

[bdraco]

Improve performance of _serialize_headers by ~23%

• benchmark Add benchmark for serializing headers #9604
• verify noexcept is safe -- seems like it should be -- removed, it was the dict iter loop that was slow anyways

[codspeed-hq]

CodSpeed Performance Report

Merging #9603 will improve performances by 23.62%

_{Comparing _serialize_headers (76c7a43) with master (898aa28)}

Summary

⚡ 1 improvements
✅ 7 untouched benchmarks

Benchmarks breakdown

	Benchmark	master	_serialize_headers	Change
⚡	test_serialize_headers	1.6 ms	1.3 ms	+23.62%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.63%. Comparing base (898aa28) to head (76c7a43).
Report is 594 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9603   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files         114      114           
  Lines       35370    35370           
  Branches     4195     4195           
=======================================
  Hits        34887    34887           
  Misses        323      323           
  Partials      160      160           

Flag	Coverage Δ
CI-GHA	98.51% <ø> (ø)
OS-Linux	98.19% <ø> (ø)
OS-Windows	96.09% <ø> (ø)
OS-macOS	97.40% <ø> (ø)
Py-3.10.11	97.26% <ø> (ø)
Py-3.10.15	97.67% <ø> (ø)
Py-3.11.10	97.73% <ø> (-0.01%)	⬇️
Py-3.11.9	97.33% <ø> (-0.02%)	⬇️
Py-3.12.7	98.22% <ø> (ø)
Py-3.13.0	98.21% <ø> (ø)
Py-3.9.13	97.17% <ø> (-0.01%)	⬇️
Py-3.9.20	97.58% <ø> (ø)
Py-pypy7.3.16	97.19% <ø> (ø)
VM-macos	97.40% <ø> (ø)
VM-ubuntu	98.19% <ø> (ø)
VM-windows	96.09% <ø> (ø)

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[patchback]

Backport to 3.10: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.10/4108ca51c10df4bf58cb5faf13cf81816a29a15d/pr-9603

Backported as #9626

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.11: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.11/4108ca51c10df4bf58cb5faf13cf81816a29a15d/pr-9603

Backported as #9627

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.