[PR #10014/50d23aee backport][3.11] Improve performance of serializing headers

[patchback]

This is a backport of PR #10014 as merged into master (50d23ae).

• Avoids ref counting in to_str since Cython can use a fast Py_TYPE check
• Only call to_str once per each key and value
• inline the CRLF check since its only used in one place now

[codspeed-hq]

CodSpeed Performance Report

Merging #10016 will improve performances by 28.25%

_{Comparing patchback/backports/3.11/50d23aeec46295ef3490770245966505d5e9de0c/pr-10014 (fd7989d) with 3.11 (5bcf07d)}

Summary

⚡ 1 improvements
✅ 42 untouched benchmarks

Benchmarks breakdown

	Benchmark	3.11	patchback/backports/3.11/50d23aeec46295ef3490770245966505d5e9de0c/pr-10014	Change
⚡	test_serialize_headers	1,280.8 µs	998.6 µs	+28.25%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (5bcf07d) to head (fd7989d).
Report is 1 commits behind head on 3.11.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.11   #10016   +/-   ##
=======================================
  Coverage   98.03%   98.03%           
=======================================
  Files         122      122           
  Lines       36890    36890           
  Branches     4441     4441           
=======================================
  Hits        36164    36164           
  Misses        543      543           
  Partials      183      183           

Flag	Coverage Δ
CI-GHA	97.92% <ø> (ø)
OS-Linux	97.63% <ø> (+<0.01%)	⬆️
OS-Windows	94.52% <ø> (ø)
OS-macOS	96.74% <ø> (ø)
Py-3.10.11	96.59% <ø> (ø)
Py-3.10.15	97.17% <ø> (ø)
Py-3.11.10	97.21% <ø> (ø)
Py-3.11.9	96.67% <ø> (-0.01%)	⬇️
Py-3.12.7	97.70% <ø> (ø)
Py-3.13.0	97.68% <ø> (-0.01%)	⬇️
Py-3.9.13	96.52% <ø> (-0.01%)	⬇️
Py-3.9.20	97.09% <ø> (-0.01%)	⬇️
Py-pypy7.3.16	96.67% <ø> (+<0.01%)	⬆️
VM-macos	96.74% <ø> (ø)
VM-ubuntu	97.63% <ø> (+<0.01%)	⬆️
VM-windows	94.52% <ø> (ø)

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