feat: create scanpy2 extra for dependencies

[ilan-gold]

I noticed that the scanpy 2 settings are not tested in general - this seems like an oversight maybe? Or?

• Closes New scanpy 2.0 optional dependency extra #4051
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.61%. Comparing base (3fb43a0) to head (929868d).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/_settings/presets.py	75.00%	3 Missing ⚠️
src/scanpy/_utils/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4055      +/-   ##
==========================================
+ Coverage   78.50%   78.61%   +0.11%     
==========================================
  Files         117      117              
  Lines       12750    12743       -7     
==========================================
+ Hits        10009    10018       +9     
+ Misses       2741     2725      -16     

Flag	Coverage Δ
hatch-test.low-vers	77.91% <77.77%> (+0.11%)	⬆️
hatch-test.pre	78.49% <77.77%> (+0.11%)	⬆️

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

Files with missing lines	Coverage Δ
src/scanpy/_settings/__init__.py	86.73% <100.00%> (+0.19%)	⬆️
src/scanpy/_utils/__init__.py	75.05% <0.00%> (-0.24%)	⬇️
src/scanpy/_settings/presets.py	89.38% <75.00%> (-1.71%)	⬇️

... and 2 files with indirect coverage changes