Update to python 3.9

[ivirshup]

Supersedes #1117

Closes #1116

This is a smaller version of #1117 that implements the rules when it was last reviewed plus a few new ones.

The big thing is that it bumps the minimum python version to 3.9

[codecov]

Codecov Report

Merging #1125 (857f5e4) into main (36cd7b9) will decrease coverage by 32.65%.
The diff coverage is 89.66%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1125       +/-   ##
===========================================
- Coverage   85.09%   52.44%   -32.65%     
===========================================
  Files          36       36               
  Lines        5287     5335       +48     
===========================================
- Hits         4499     2798     -1701     
- Misses        788     2537     +1749     

Flag	Coverage Δ
gpu-tests	52.44% <89.66%> (-0.03%)	⬇️

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

Files Changed	Coverage Δ
anndata/experimental/_dispatch_io.py	42.85% <33.33%> (-57.15%)	⬇️
anndata/_core/raw.py	65.97% <61.53%> (-17.60%)	⬇️
anndata/experimental/pytorch/_annloader.py	22.00% <71.42%> (+1.38%)	⬆️
anndata/_core/file_backing.py	56.84% <75.00%> (-33.49%)	⬇️
anndata/_core/views.py	79.12% <75.00%> (-10.63%)	⬇️
anndata/_core/access.py	70.00% <83.33%> (-2.23%)	⬇️
anndata/_io/specs/registry.py	82.09% <85.71%> (-14.18%)	⬇️
anndata/utils.py	41.42% <85.71%> (-43.52%)	⬇️
anndata/_io/specs/methods.py	48.29% <87.50%> (-39.91%)	⬇️
anndata/_io/write.py	19.31% <87.50%> (-57.16%)	⬇️
... and 26 more

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