Include downstream tests in source distributions - #12

The strategy configuration was canceled because "tests.Ubuntu_3_7" failed

The version '3.9' with architecture 'x64' was not found for Ubuntu 26.04. The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v3, actions/setup-python@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/