Improve performance of serializing headers

[bdraco]

• 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 #10014 will improve performances by 27.66%

_{Comparing serialize_headers (d7dcf42) with master (c984a44)}

Summary

⚡ 1 improvements
✅ 42 untouched benchmarks

Benchmarks breakdown

	Benchmark	master	serialize_headers	Change
⚡	test_serialize_headers	1.3 ms	1 ms	+27.66%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.73%. Comparing base (c984a44) to head (d7dcf42).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10014   +/-   ##
=======================================
  Coverage   98.73%   98.73%           
=======================================
  Files         121      121           
  Lines       36727    36727           
  Branches     4384     4384           
=======================================
  Hits        36261    36261           
  Misses        314      314           
  Partials      152      152           

Flag	Coverage Δ
CI-GHA	98.61% <ø> (ø)
OS-Linux	98.30% <ø> (ø)
OS-Windows	96.13% <ø> (ø)
OS-macOS	97.40% <ø> (ø)
Py-3.10.11	97.25% <ø> (+<0.01%)	⬆️
Py-3.10.15	97.79% <ø> (ø)
Py-3.11.10	97.83% <ø> (ø)
Py-3.11.9	97.30% <ø> (-0.01%)	⬇️
Py-3.12.7	98.36% <ø> (ø)
Py-3.13.0	98.33% <ø> (-0.02%)	⬇️
Py-3.9.13	97.17% <ø> (ø)
Py-3.9.20	97.71% <ø> (ø)
Py-pypy7.3.16	97.32% <ø> (+<0.01%)	⬆️
VM-macos	97.40% <ø> (ø)
VM-ubuntu	98.30% <ø> (ø)
VM-windows	96.13% <ø> (ø)

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[patchback]

Backport to 3.10: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.10/50d23aeec46295ef3490770245966505d5e9de0c/pr-10014

Backported as #10015

🤖 @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/3.11/50d23aeec46295ef3490770245966505d5e9de0c/pr-10014

Backported as #10016

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

[patchback]

Backport to 3.12: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.12/50d23aeec46295ef3490770245966505d5e9de0c/pr-10014

Backported as #10017

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