I'm looking for a simple to use GUI for my FOSS python project.
I have tried tkinter which is, uh, usable but seriously oldish? Good point seems to be it's basically inbuilt in python so not hard to distribute.
PyQt is on the heavy end, I just need windows, scrollbars and buttons (basically, see below), also I wonder about the license of Qt (it's always a PITA when trying to do C++ Qt) and also what you must package when you distribute the soft.
Must haves:
Frames (I need two independent lists of files and another with global info)
Text, buttons & colors, an "open file" dialog. Editable text field.
I've historically used PySide (the free-license version of Qt) but for simple stuff like you're looking for, you might get some mileage out of the Toga GUI toolkit. It's relatively new, but promising.
I've actually been pretty impressed with the whole suite of BeeWare stuff in my informal testing so far; it's a nice little bundle of tools. (Specifically I'm interested most in their distribution approach; building Python apps for distribution is a giant fucking pain, but this group seems to have improved the experience significantly.)
I don't know if this something you're deliberately trying to avoid. Apologies if you are, and I've missed the point, but
I gave up on doing anything in TK years ago. For all the effort to make stuff work in it, you might as well just use flask and have a HTML frontend. That way, you know it's going to work on everything and includes remote access as a bonus.
Edit: for a lot more power with a little bit more learning curve, look at fastapi.
It use to be open source. When they launched V5 this year, they decided to take down V4 and make the project closed source. You can still find forked repos of V4 on github that can be used freely.
I'm only mentioning this because it's not been mentioned in any other comments but there is a Python implementation for the CEF (Chrome Embedded Framework). It let's you write your front end in HTML/CSS and JS while letting you call back to a Python backend. You can use any existing JS framework to do your styling (offering the most flexibility) while keeping business logic in Python. It's not exactly what you were asking for, however you mentioned in a different comment thread that tkinter looked outdated, so thought I'd mention it.