Thanks for the suggestions so far, everyone — I will definitely have a look and give them a try!
I think I owe you a slightly longer explanation of what I meant by "state" or "game state". Certainly, I am going to need to implement persistence of the game state the player cares about, such as their actual game progress, that should be able to survive restarting the game.
For now, however, I am only working on the UI framework and this is where being able to reload just the bits I actually changed is crucial for rapid development. For example, this UI features dialog windows that may pop from one another up to 5 levels deep or more. Without proper reloading, I have to go through the whole sequence of opening these windows one after another for any small adjustment I make in the code of the deepest-nested one to finally see the effects of said changes.
I actually started prototyping this game (engine) in Python, but I've outgrown its limited reloading capabilities rather quickly and was continuing purely by inertia up until now... I can share the link to the project, if there is interest and it's not against the rules here. 