Bump actions/download-artifact to v4

[Stoops-ML]

Fixes [Publish Python 🐍 distribution 📦 to PyPI](https://github.com/poliastro/czml3/actions/runs/9713111295/job/26809222219#logs

EDIT: deleted the workflow so above link doesn't work, so here's a picture:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.25%. Comparing base (1a1a364) to head (0288a3f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #144   +/-   ##
=======================================
  Coverage   99.25%   99.25%           
=======================================
  Files          12       12           
  Lines         805      805           
=======================================
  Hits          799      799           
  Misses          6        6           

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