Fix an error in `PyListModel.__setitem__`
Fix an error in `PyListModel.__setitem__`

Before it would overwrite elements which were not supposed to be
overwritten.

cheral/orange3,qPCR4vir/orange3,qPCR4vir/orange3,qPCR4vir/orange3,qPCR4vir/orange3,qPCR4vir/orange3,cheral/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,cheral/orange3,cheral/orange3