Add python-proxy-headers to third_party.rst

[proxymesh]

for aiohttp_proxy extension

What do these changes do?

Add python-proxy-headers to third party library list

Are there changes in behavior for the user?

No

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

No

Checklist

• Documentation reflects the changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.74%. Comparing base (63716df) to head (e910b4f).
⚠️ Report is 242 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11865   +/-   ##
=======================================
  Coverage   98.74%   98.74%           
=======================================
  Files         127      127           
  Lines       44171    44171           
  Branches     2342     2342           
=======================================
  Hits        43618    43618           
  Misses        392      392           
  Partials      161      161           

Flag	Coverage Δ
CI-GHA	98.60% <ø> (ø)
OS-Linux	98.34% <ø> (ø)
OS-Windows	96.69% <ø> (+<0.01%)	⬆️
OS-macOS	97.56% <ø> (-0.01%)	⬇️
Py-3.10.11	97.10% <ø> (ø)
Py-3.10.19	97.60% <ø> (ø)
Py-3.11.14	97.80% <ø> (-0.01%)	⬇️
Py-3.11.9	97.31% <ø> (ø)
Py-3.12.10	97.41% <ø> (-0.01%)	⬇️
Py-3.12.12	97.91% <ø> (-0.01%)	⬇️
Py-3.13.11	98.16% <ø> (+<0.01%)	⬆️
Py-3.14.2	98.17% <ø> (ø)
Py-3.14.2t	97.25% <ø> (+<0.01%)	⬆️
Py-pypy3.11.13-7.3.20	97.41% <ø> (-0.01%)	⬇️
VM-macos	97.56% <ø> (-0.01%)	⬇️
VM-ubuntu	98.34% <ø> (ø)
VM-windows	96.69% <ø> (+<0.01%)	⬆️

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[codspeed]

CodSpeed Performance Report

Merging #11865 will not alter performance

_{Comparing proxymesh:patch-1 (e910b4f) with master (63716df)¹}

Summary

✅ 59 untouched

Footnotes

1. No successful run was found on master (e730788) during the generation of this report, so 63716df was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[patchback]

Backport to 3.14: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.14/963ca76708cd4bc52e726db10fd296c2025d003e/pr-11865

Backported as #11866

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