Replace custom _add_slots decorator with dataclasses slots=True

[zsol]

Replace custom _add_slots decorator with dataclasses slots=True

Dataclasses has a built-in slots generation feature since Python 3.10.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.04%. Comparing base (8a19d05) to head (2feb8ea).
Report is 2 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1113      +/-   ##
==========================================
- Coverage   91.12%   91.04%   -0.09%     
==========================================
  Files         256      254       -2     
  Lines       26578    26313     -265     
==========================================
- Hits        24220    23956     -264     
+ Misses       2358     2357       -1     

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