Deprecate louvain

[flying-sheep]

• Fixes Louvain package is deprecated in favor of leidenalg #1379, fixes Integration Testing CI Test Failure on python 3.12 #3651
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.14%. Comparing base (89780ec) to head (283bb9f).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3658      +/-   ##
==========================================
- Coverage   76.14%   76.14%   -0.01%     
==========================================
  Files         114      114              
  Lines       12741    12452     -289     
==========================================
- Hits         9702     9481     -221     
+ Misses       3039     2971      -68     

Files with missing lines	Coverage Δ
src/scanpy/tools/_leiden.py	93.93% <ø> (+5.05%)	⬆️
src/scanpy/tools/_louvain.py	100.00% <100.00%> (+41.66%)	⬆️

... and 7 files with indirect coverage changes

[ilan-gold]

Linked issue suggests delaying louvain imports until later, can we do that as well?

[flying-sheep]

huh, I found a way to do it, but it looks weird!