[PR #8033/5424c534 backport][3.9] Small cleanups to WebSocketWriter

[patchback]

This is a backport of PR #8033 as merged into master (5424c53).

What do these changes do?

• Add missing typing for random
• Avoid duplicate len() calls

Are there changes in behavior for the user?

no

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (41968f8) 97.39% compared to head (0099136) 97.39%.
Report is 1 commits behind head on 3.9.

Additional details and impacted files

@@           Coverage Diff           @@
##              3.9    #8036   +/-   ##
=======================================
  Coverage   97.39%   97.39%           
=======================================
  Files         108      108           
  Lines       32496    32496           
  Branches     3848     3848           
=======================================
  Hits        31648    31648           
  Misses        646      646           
  Partials      202      202           

Flag	Coverage Δ
CI-GHA	97.30% <100.00%> (ø)
OS-Linux	96.99% <100.00%> (ø)
OS-Windows	94.48% <100.00%> (+<0.01%)	⬆️
OS-macOS	96.61% <100.00%> (-0.20%)	⬇️
Py-3.10.11	94.37% <100.00%> (ø)
Py-3.10.13	96.77% <100.00%> (ø)
Py-3.11.7	96.50% <100.00%> (-0.04%)	⬇️
Py-3.12.1	96.55% <100.00%> (-0.04%)	⬇️
Py-3.8.10	94.36% <100.00%> (+<0.01%)	⬆️
Py-3.8.18	96.70% <100.00%> (ø)
Py-3.9.13	94.37% <100.00%> (+<0.01%)	⬆️
Py-3.9.18	96.76% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.14	?
Py-pypy7.3.15	96.30% <100.00%> (?)
VM-macos	96.61% <100.00%> (-0.20%)	⬇️
VM-ubuntu	96.99% <100.00%> (ø)
VM-windows	94.48% <100.00%> (+<0.01%)	⬆️

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