rewrite check_files_md5 in pure Python - #643

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: canonical/has-signed-canonical-cla@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/