ci: add Python 3.15 to the test matrix - #196

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@08eba0b27e820071cde6df949e0beb9ba4906955, actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065, astral-sh/setup-uv@d0cc045d04ccac9d8b7881df0226f9e82c39688e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/