test: fix matplotlib deprecations

[flying-sheep]

Waiting for matplotlib/matplotlib#31575 to solve the remaining stuff

• Closes #
• Tests included or not required because:

• Release notes not necessary because: tests only

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.22%. Comparing base (463b0bb) to head (080f535).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #4091       +/-   ##
=========================================
+ Coverage      0   79.22%   +79.22%     
=========================================
  Files         0      119      +119     
  Lines         0    12667    +12667     
=========================================
+ Hits          0    10036    +10036     
- Misses        0     2631     +2631     

Flag	Coverage Δ
hatch-test.low-vers	78.50% <100.00%> (?)
hatch-test.pre	79.11% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/plotting/_tools/paga.py	64.26% <ø> (ø)
src/scanpy/plotting/_tools/scatterplots.py	83.41% <100.00%> (ø)

... and 117 files with indirect coverage changes