feat: microoptimize _LRUCacheWrapper._task_done_callback

[BobTheBuidler]

This PR just microoptimizes attribute lookups in _LRUCacheWrapper._task_done_callback to streamline call speed.

Checklist

• I think the code is well written
• Unit tests for the changes exist

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.23%. Comparing base (2913fe2) to head (4d5e3f0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #684   +/-   ##
=======================================
  Coverage   97.22%   97.23%           
=======================================
  Files          12       12           
  Lines         793      795    +2     
  Branches       41       41           
=======================================
+ Hits          771      773    +2     
  Misses         20       20           
  Partials        2        2           

Flag	Coverage Δ
unit	95.84% <100.00%> (+0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #684 will not alter performance

_{Comparing BobTheBuidler:callback (4d5e3f0) with master (2913fe2)}

Summary

✅ 39 untouched