python-pyqt: use 'depends on' rather than 'select' for Qt

When pyqt is used it's obvious that qt needs to be selected and
configured by the user, hence we enforce it by making pyqt depending on
qt, rather than selecting it and depending only on the qt requirements.

Signed-off-by: Andreas Naumann <anaumann@ultratronik.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>