[PR #9554/ec19bfa7 backport][3.11] Optimize WebSocketReader for 2 byte length case

[patchback]

This is a backport of PR #9554 as merged into master (ec19bfa).

Optimize WebSocketReader for 2 byte length case. The simple 2 byte length unpack can be implemented almost entirely in native code.

The profile showed a lot of time in unpack_from so this should have speed this up a bit more

[codspeed-hq]

CodSpeed Performance Report

Merging #9555 will improve performances by 13.01%

_{Comparing patchback/backports/3.11/ec19bfa753ff3cd1869c2e1da4f55ef561897fbb/pr-9554 (8feef87) with 3.11 (ecb6490)}

Summary

⚡ 1 improvements
✅ 3 untouched benchmarks

Benchmarks breakdown

	Benchmark	3.11	patchback/backports/3.11/ec19bfa753ff3cd1869c2e1da4f55ef561897fbb/pr-9554	Change
⚡	test_read_one_hundred_websocket_text_messages[pyloop]	645.8 µs	571.5 µs	+13.01%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.84%. Comparing base (ecb6490) to head (8feef87).
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    #9555   +/-   ##
=======================================
  Coverage   97.84%   97.84%           
=======================================
  Files         114      114           
  Lines       35313    35313           
  Branches     4236     4236           
=======================================
  Hits        34552    34552           
  Misses        570      570           
  Partials      191      191           

Flag	Coverage Δ
CI-GHA	97.72% <100.00%> (ø)
OS-Linux	97.42% <100.00%> (+<0.01%)	⬆️
OS-Windows	94.32% <25.00%> (-0.01%)	⬇️
OS-macOS	96.64% <25.00%> (-0.01%)	⬇️
Py-3.10.11	96.50% <25.00%> (-0.01%)	⬇️
Py-3.10.15	96.91% <100.00%> (ø)
Py-3.11.10	96.99% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	96.59% <25.00%> (+<0.01%)	⬆️
Py-3.12.7	97.45% <100.00%> (ø)
Py-3.13.0	97.44% <100.00%> (ø)
Py-3.9.13	96.42% <25.00%> (ø)
Py-3.9.20	96.83% <100.00%> (ø)
Py-pypy7.3.16	96.45% <100.00%> (ø)
VM-macos	96.64% <25.00%> (-0.01%)	⬇️
VM-ubuntu	97.42% <100.00%> (+<0.01%)	⬆️
VM-windows	94.32% <25.00%> (-0.01%)	⬇️

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