Scipy 1.15 compat, some test refactors

[flying-sheep]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.65%. Comparing base (7e3dd15) to head (d5ce77b).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3409      +/-   ##
==========================================
+ Coverage   76.53%   76.65%   +0.11%     
==========================================
  Files         111      112       +1     
  Lines       12881    12931      +50     
==========================================
+ Hits         9859     9912      +53     
+ Misses       3022     3019       -3     

Files with missing lines	Coverage Δ
src/scanpy/tools/_rank_genes_groups.py	94.52% <100.00%> (ø)

... and 5 files with indirect coverage changes