Backport PR #4141 on branch 1.12.x (perf: single-pass _sparse_nanmean (#1894))

[meeseeksmachine]

Backport PR #4141: perf: single-pass _sparse_nanmean (#1894)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.54%. Comparing base (d364f06) to head (6f27e84).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           1.12.x    #4177      +/-   ##
==========================================
- Coverage   78.54%   78.54%   -0.01%     
==========================================
  Files         121      121              
  Lines       12650    12648       -2     
==========================================
- Hits         9936     9934       -2     
  Misses       2714     2714              

Flag	Coverage Δ
hatch-test.low-vers	77.79% <100.00%> (-0.01%)	⬇️
hatch-test.pre	78.43% <100.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/scanpy/tools/_score_genes.py	86.53% <100.00%> (-0.26%)	⬇️