My understanding is that Python has bindings for a handful popular toolkits, the most notable being GTK, QT and TK.
QT has Python bindings provided by the package "python3-pyside". It's documented in Learning Python With Raspberry Pi
A friend recently mentioned PyGObject to me as a ways of building GTK applications in Python. That's covered here
I'm just getting started on GUIs myself so I couldn't tell you which toolkits to go for or reasons why to pick one toolkit over another. Hopefully somebody with more experience will chime in...
Hope this info at least gets you started.