Add Python 3.15 to CI - #38

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/setup-python@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

numpy 1.23 support can be dropped per PHEP 3 - Version 1.23 is older than the minimum required version (numpy>=2.2)

Python 3.9 support can be dropped per PHEP 3 - Python 3.9 is older than the minimum required version (3.13)