Doing some like a=qt.QPushButton() is already the python interface that you can use in Slicer to interact with Qt which is C++ based. Slicer uses PythonQt and not PyQt. They are similar in syntax, but with different licensing.
How are you currently connecting the button press to updating the label?
I generally connect actions using the newer style of
a=qt.QPushButton()
a.clicked.connect(slotName)