[PR #10038/6f4e9615 backport][3.11] Small speed up to StreamWriter.__init__

[patchback]

This is a backport of PR #10038 as merged into master (6f4e961).

Use classvar defaults to avoid __init__ overhead. AbstractStreamWriter already does this so we can drop a few duplicates as well.

[codspeed-hq]

CodSpeed Performance Report

Merging #10039 will not alter performance

_{Comparing patchback/backports/3.11/6f4e9615d74c7d4935c11a2c9497cf6aae7149f3/pr-10038 (91d0bcb) with 3.11 (3d2b829)}

Summary

✅ 44 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.03%. Comparing base (3d2b829) to head (91d0bcb).
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   #10039      +/-   ##
==========================================
- Coverage   98.03%   98.03%   -0.01%     
==========================================
  Files         122      122              
  Lines       36933    36930       -3     
  Branches     4449     4449              
==========================================
- Hits        36208    36205       -3     
  Misses        543      543              
  Partials      182      182              

Flag	Coverage Δ
CI-GHA	97.92% <100.00%> (-0.01%)	⬇️
OS-Linux	97.63% <100.00%> (-0.01%)	⬇️
OS-Windows	94.53% <100.00%> (-0.01%)	⬇️
OS-macOS	96.74% <100.00%> (-0.01%)	⬇️
Py-3.10.11	96.60% <100.00%> (-0.01%)	⬇️
Py-3.10.15	97.18% <100.00%> (+0.03%)	⬆️
Py-3.11.10	97.25% <100.00%> (+0.04%)	⬆️
Py-3.11.9	96.67% <100.00%> (+<0.01%)	⬆️
Py-3.12.7	97.70% <100.00%> (+<0.01%)	⬆️
Py-3.13.0	97.68% <100.00%> (-0.01%)	⬇️
Py-3.9.13	96.53% <100.00%> (-0.01%)	⬇️
Py-3.9.20	97.11% <100.00%> (-0.01%)	⬇️
Py-pypy7.3.16	96.68% <100.00%> (+<0.01%)	⬆️
VM-macos	96.74% <100.00%> (-0.01%)	⬇️
VM-ubuntu	97.63% <100.00%> (-0.01%)	⬇️
VM-windows	94.53% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.