There's LGPL PySide (http://qt-project.org/wiki/PySide) which should be suitable for commercial projects under LGPL, but PySide is Qt4 only. There is also PyQt (http://www.riverbankcomputing.co.uk/software/pyqt/license) with Qt5 support, but its free license is GPL-only, so if you're not doing open source, it's not going to work, unless you buy their commercial license.
BTW, Zeal's original author here - thanks for mentioning Zeal and good luck with your project!
As some personal advice, I wouldn't try Kivy for such content-oriented desktop app. I've tried briefly using it once for doing a very simple application, but it seemed much harder to implement usual desktop-like widgets. Overall it looks like it's good for graphics-rich and custom-rendered applications.
BTW, Zeal's original author here - thanks for mentioning Zeal and good luck with your project!
As some personal advice, I wouldn't try Kivy for such content-oriented desktop app. I've tried briefly using it once for doing a very simple application, but it seemed much harder to implement usual desktop-like widgets. Overall it looks like it's good for graphics-rich and custom-rendered applications.
You'll also need some HTML rendering engine for docsets. While there are Kivy projects like https://github.com/kivy-garden/garden.cefpython or https://github.com/kivy/kivy-berkelium aiming to provide exactly that, they don't seem very mature.
However, I don't claim to be Kivy expert at all, and maybe you'll have good experience with it.