As it was mentioned in another comment, its hard to find a link to the source code of this, or even to get the information, whether this is open source, and what licence. (Answer: https://github.com/mu-editor/mu/, licence is GPLv3). For some people, this is some of the most important question before they would try out a new app.
Also, one detail which was hard to find, but interesting for me: This is build using PyQt5.
About the MacOSX package: It's about 430 MB. This is a lot for such a simple editor. Is this just because of PyQt5 which is bundled together with it? Isn't it possible to strip that down? I would have expected something like 5 MB or so.
Also, another question: Is it possible to have this bundled into another existing app? E.g. I have an app where the user could write simple Python scripts to automate something (e.g. the smart playlist logic in a music player), and I was wondering whether I can have a simple editor just inside the app. However, your licence is problematic for this usage, as my apps usually have a BSD or MIT licence or similar. Maybe consider to use LGPL?