Tooling updates

[mattwthompson]

• Migrate to Pixi
• Restructure to support multiple Python versions/Pixi environments
• Update Python project metadata, minimum Python version
• Update linters, actions
• Add Dependabot
• Keep docs building - there weren't really docs in this repo, so I removed the mostly-empty setup. We can add docs back later
• Try Python 3.13
• Try type-checking
• Remove old tooling files

Description

Big grab bag of sweeping tooling changes, should have minimal impact on source code or usage

Status

• Ready to go

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.66%. Comparing base (cbe3db1) to head (14e986b).

Files with missing lines	Patch %	Lines
descent/train.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   97.65%   97.66%           
=======================================
  Files          11       11           
  Lines        1068     1071    +3     
=======================================
+ Hits         1043     1046    +3     
  Misses         25       25           

Flag	Coverage Δ
unittests	97.66% <92.85%> (+<0.01%)	⬆️

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