This is a wrapper around the Python Tkinter and GTK+ 3 bindings. You can write code using BananaGUI, and then run the same code using any of these toolkits. BananaGUI may feature PyQt5 support later.

Update 2021: I recommend moving to Logseq, because it has a better performance with many notes, and the concept of having a network instead of a tree is easier to organize. Half a year of research ...