First thing: a good IDE would allow you to replace QFileDialog with your own class pretty easily and would be the right thing to do with regard to longterm maintenance and peace of mind.

Now since it's python, you can apply some black magic. The brutal way to do it would be something like:

import QtWidgets

class MyCoolFileDialog(QtWidgets.QWidget):
    ....
QtWidgets.QFileDialog = MyCoolFileDialog

I am guessing that you might just be using one of the static method of QFileDialog, if that's the case, then just replace the static method rather that the whole class.

The right term is monkey patching.

You definitely want to put some warning somewhere about what you are doing here and document it properly.