After some more searching I came across the answer. This helped me, to add the follow option
to the openFileDialog
fname, _ = QFileDialog.getOpenFileName(self, 'Open file',
'', '',
options=QFileDialog.DontUseNativeDialog)
This answer was found by @wagnerpeer on the following SO question
No files visible in the QFileDialog window
EDIT: @ekhumoro his comment below mine is very important and correct. This solution might only be needed when working from within an IDE