Maybe you could try a Python to JS compiler like Transcrypt or Pyjs with Electron and co. Been quite a few years since I used Python so I don't know what's popular there right now for cross platfom desktop development.

libui seems to be getting popular as a cross platform toolkit with bindings for tons of languages including Python here: https://github.com/joaoventura/pylibui

Haven't tried it myself though so no idea if it's pythonic/idiomatic or just direct bindings. Have to give it a try one of these days, right now I'm using GTK3 directly.