Ah, I should've mentioned that. 
My Bullseye uses python3 by default whereas Buster still has python2 in use.
So run it with python3 on buster (or maybe change the first line to python3 env. 
)
and keep an eye open if the module is installed to the right python version.
That's something we direly need to do too: Change over to python3, also on the current Buster based Elive version.
Can be done fairly easitly (I think) .... Like I said it was a quicky 
but by the look of things it's fairly do-able to copy your script one-on-one and only use the python-efl bindings for the GUI.
It can serve as a template for future GUI pop-ups and widgets. EFL can do a lot more than zenity has available.
- Let's get messy:
All in all we can have it running from a shell script, setting all the variables already defined for zenity scripts... and then call 'python-efl' from that script and use the shell again from inside there.
In a python script we can ultimately define anything we want.
I think this format can be fairly useful for all the elive-tools.
It's as simple as creating a shell script and calling it from the python-efl GUI once a button is clicked.
Frankly I've only discovered this (python) GUI option as of today and I'm surprised how easy it is to apply. 
Note(or tip):
'mu-editor' is a very nice way to run and test python apps .... it's in the repos. 