Slicer uses PythonQt library for Python wrapping of Qt. PyQt is not free to use (it has GPL/commercial license), so we have never even considered using it. Python for Qt is very popular, as it is provided by the Qt company, but PythonQt has many advantages - most importantly that it is intended for applications that embed Python.
Anyway, Slicer imports all Qt packages in qt namespace. For example, QFile class in qtcore can be accessed as qt.QFile()