Replace set with deque for pending tasks

[asvetlov]

No description provided.

[codspeed-hq]

CodSpeed Performance Report

Merging #707 will not alter performance

_{Comparing deque (fd89b26) with master (aa8353c)}

Summary

✅ 4 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.49%. Comparing base (aa8353c) to head (fd89b26).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #707   +/-   ##
=======================================
  Coverage   94.49%   94.49%           
=======================================
  Files           5        5           
  Lines        1363     1363           
  Branches      109      109           
=======================================
  Hits         1288     1288           
  Misses         50       50           
  Partials       25       25           

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