python-pyqt5: use 'depends on' rather than 'select' for Qt5

When pyqt5 is used it's obvious that qt5 needs to be selected and
configured by the user, hence we enforce it by making pyqt5 depending on
qt5, rather than selecting it and depending only on the qt5
requirements.

Signed-off-by: Andreas Naumann <anaumann@ultratronik.de>
Signed-off-by: Zoltan Gyarmati <mr.zoltan.gyarmati@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>