Backport PR #2970 on branch 1.2.x (Drop Python 3.9)

[meeseeksmachine]

Backport PR #2970: Drop Python 3.9

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.28%. Comparing base (7b5e8e2) to head (ac94342).
Report is 1 commits behind head on 1.2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##            1.2.x    #2972      +/-   ##
==========================================
- Coverage   85.30%   85.28%   -0.03%     
==========================================
  Files         167      167              
  Lines       14369    14349      -20     
==========================================
- Hits        12258    12238      -20     
  Misses       2111     2111              

Files with missing lines	Coverage Δ
src/scvi/_settings.py	84.40% <100.00%> (ø)
src/scvi/_types.py	100.00% <100.00%> (ø)
src/scvi/autotune/_experiment.py	89.38% <100.00%> (ø)
src/scvi/data/_anntorchdataset.py	93.33% <100.00%> (ø)
src/scvi/data/_built_in_data/_cellxgene.py	31.81% <ø> (-2.97%)	⬇️
src/scvi/data/_built_in_data/_pbmc.py	75.38% <100.00%> (ø)
src/scvi/data/_built_in_data/_synthetic.py	100.00% <ø> (ø)
src/scvi/data/_compat.py	82.95% <100.00%> (-0.20%)	⬇️
src/scvi/data/_download.py	86.84% <100.00%> (-0.34%)	⬇️
src/scvi/data/_preprocessing.py	78.57% <100.00%> (-0.13%)	⬇️
... and 55 more