Switch to hatch and Python 3.9 minimum

[flying-sheep]

Combines the non-linter changes from scverse/anndata#1001 and scverse/anndata#1125

The linter changes like https://github.com/scverse/anndata/pull/1125/files#diff-50c86b7ed8ac2cf95bd48334961bf0530cdc77b5a56f852c5c61b89d735fd711 can be done later too

[codecov]

Codecov Report

Merging #2695 (1275fd8) into master (373c585) will decrease coverage by 0.03%.
The diff coverage is 82.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2695      +/-   ##
==========================================
- Coverage   72.01%   71.99%   -0.03%     
==========================================
  Files         108      107       -1     
  Lines       11934    11914      -20     
==========================================
- Hits         8594     8577      -17     
+ Misses       3340     3337       -3     

Files	Coverage Δ
scanpy/neighbors/__init__.py	79.72% <100.00%> (+0.16%)	⬆️
scanpy/__init__.py	81.48% <81.48%> (-18.52%)	⬇️

[Zethson]

LGTM although I got triggered with the replacement of a relative file link to LICENSE and a hard coded license :) But I don't expect this to (ever) change.

[flying-sheep]

replacement of a relative file link to LICENSE and a hard coded license

Ah, but that’s not what that is. It’s a switch to PEP 639, which says:

If the license-files key is not present and not explicitly marked as dynamic, tools MUST assume a default value of the following:

license-files.globs = ["LICEN[CS]E*", "COPYING*", "NOTICE*", "AUTHORS*"]

So nothing is lost, the file still gets included.