fix #87; compatibility with pandas 3.0

[chenyangkang]

Addressing the changes in Pandas 3.0 compared to 2.X on how the indexes are handled in groupby.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.55%. Comparing base (5810f12) to head (f8ff490).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #88   +/-   ##
=======================================
  Coverage   90.55%   90.55%           
=======================================
  Files          40       40           
  Lines        3589     3589           
=======================================
  Hits         3250     3250           
  Misses        339      339           

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