I wish PySide (and/or PySide2) would work with PyPy. It looks like (after a cursory search) some version of Gtk can be used but I prefer Qt over Gtk.
Not sure about PySide2..
Therefore until and unless PyPy has usable Qt bindings (e.g. PySide or PyQt), then unfortunately I need to stick with CPython :/
Here is the only PyPy related issue I found for PySide2: https://bugreports.qt.io/browse/PYSIDE-535
PyQt doesn't have a bug tracker ¯\_(ツ)_/¯