Provide a base app for PySide flatpak development

It was brought to our attention, that people developing KDE applications have the option to use Python, through PyQt [1] [2].
To offer also the alternative of having PySide based applications, we would like to create a base app package so people can get started with PySide and Flatpak easily.

Initially, this has the KDE community in mind, but we understand that by providing this flatpak package, other communities and projects can benefit.

[1] https://develop.kde.org/docs/getting-started/python/
[2] https://github.com/flathub/com.riverbankcomputing.PyQt.BaseApp