Build CPython 3.15 wheels - #148

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@eef61447b9ff4aafe5dcd4e0bbf5d482be7e7871. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/