[PR #8933/8f3b1f44 backport][3.11] Small cleanups to the websocket frame sender

[patchback]

This is a backport of PR #8933 as merged into master (8f3b1f4).

What do these changes do?

• Add some comments about what is going on in _send_frame
• Avoid fetch the len() in a few places where its a fixed value and already known

Are there changes in behavior for the user?

no

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

no

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.74%. Comparing base (be27f79) to head (7a677aa).
Report is 565 commits behind head on 3.11.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.11    #8979   +/-   ##
=======================================
  Coverage   97.74%   97.74%           
=======================================
  Files         108      108           
  Lines       34435    34438    +3     
  Branches     4085     4084    -1     
=======================================
+ Hits        33658    33661    +3     
  Misses        581      581           
  Partials      196      196           

Flag	Coverage Δ
CI-GHA	97.64% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.31% <100.00%> (-0.01%)	⬇️
OS-Windows	94.88% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.00% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	97.09% <100.00%> (+<0.01%)	⬆️
Py-3.10.14	97.03% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.25% <100.00%> (+<0.01%)	⬆️
Py-3.12.5	97.36% <100.00%> (+<0.01%)	⬆️
Py-3.9.13	97.00% <100.00%> (+<0.01%)	⬆️
Py-3.9.19	96.94% <100.00%> (+<0.01%)	⬆️
Py-pypy7.3.16	96.53% <100.00%> (+<0.01%)	⬆️
VM-macos	97.00% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.31% <100.00%> (-0.01%)	⬇️
VM-windows	94.88% <100.00%> (+<0.01%)	⬆️

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