Add Python 3.15 to CI matrix - #1037

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/