[PR #9558/0a706625 backport][3.11] Fix refactoring error from moving WebSocket mask

[patchback]

This is a backport of PR #9558 as merged into master (0a70662).

#9543 missed changing the import when the code was relocated. Fix the import and adjust the test to confirm the import works. This only affected the writer since the reader now cimports the mask code.

Performance tests did not catch it because the benchmarks do not use masking. A new benchmark will be added in #9559

[codspeed-hq]

CodSpeed Performance Report

Merging #9563 will not alter performance

_{Comparing patchback/backports/3.11/0a706625ba43e34f734156a03699fc93d2b10180/pr-9558 (5e84b73) with 3.11 (6a1b086)}

Summary

✅ 4 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.86%. Comparing base (6a1b086) to head (5e84b73).
Report is 270 commits behind head on 3.11.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             3.11    #9563      +/-   ##
==========================================
+ Coverage   97.84%   97.86%   +0.01%     
==========================================
  Files         114      114              
  Lines       35320    35321       +1     
  Branches     4236     4236              
==========================================
+ Hits        34559    34567       +8     
+ Misses        570      563       -7     
  Partials      191      191              

Flag	Coverage Δ
CI-GHA	97.74% <100.00%> (+0.01%)	⬆️
OS-Linux	97.43% <100.00%> (+0.01%)	⬆️
OS-Windows	94.34% <100.00%> (+0.02%)	⬆️
OS-macOS	96.66% <100.00%> (+0.01%)	⬆️
Py-3.10.11	96.52% <100.00%> (+0.01%)	⬆️
Py-3.10.15	96.93% <100.00%> (+0.01%)	⬆️
Py-3.11.10	97.00% <100.00%> (+0.01%)	⬆️
Py-3.11.9	96.60% <100.00%> (+0.02%)	⬆️
Py-3.12.7	97.47% <100.00%> (+0.01%)	⬆️
Py-3.13.0	97.46% <100.00%> (+0.01%)	⬆️
Py-3.9.13	96.44% <100.00%> (+0.01%)	⬆️
Py-3.9.20	96.85% <100.00%> (+0.01%)	⬆️
Py-pypy7.3.16	96.45% <0.00%> (-0.01%)	⬇️
VM-macos	96.66% <100.00%> (+0.01%)	⬆️
VM-ubuntu	97.43% <100.00%> (+0.01%)	⬆️
VM-windows	94.34% <100.00%> (+0.02%)	⬆️

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