Backport PR #4184 on branch 1.12.x (fix: inconsist variance output between in-memory and dask for groups of size 1)

[meeseeksmachine]

Backport PR #4184: fix: inconsist variance output between in-memory and dask for groups of size 1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.61%. Comparing base (362d71d) to head (ddeba09).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           1.12.x    #4185   +/-   ##
=======================================
  Coverage   78.60%   78.61%           
=======================================
  Files         121      121           
  Lines       12688    12693    +5     
=======================================
+ Hits         9973     9978    +5     
  Misses       2715     2715           

Flag	Coverage Δ
hatch-test.low-vers	77.67% <80.00%> (+<0.01%)	⬆️
hatch-test.pre	78.49% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/scanpy/get/_aggregated.py	93.53% <100.00%> (+0.12%)	⬆️