CI cleanups and README updates

[amotl]

• CI: Use vanilla Python 3.12 (no -dev)
• README: Update badges

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.45%. Comparing base (1757ec9) to head (dc92657).
Report is 2 commits behind head on main.

❗ Current head dc92657 differs from pull request most recent head 01d377a. Consider uploading reports for the commit 01d377a to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files           5        5           
  Lines         154      154           
=======================================
  Hits          147      147           
  Misses          7        7           

Flag	Coverage Δ
unittests	95.45% <ø> (ø)

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