Python 3.11.6. One for Sigil built with Qt5 (PyQt5) and one for Sigil built with Qt6 (PySide6).
Download, make sure it's executable, and use Sigil's Plugin Manager to assign it as the interpreter for plugins. Should work as far back as GLIBC 2.28 (Ubuntu 20.04-ish).
They're a bit beefy, but...