Scene items can be selectable, and scene selection refers to selecting of items. You have only one scene item (a QGraphicsProxyWidget
for displaying a table) and it is not selectable, so scene selection is always empty and cannot be changed.
You need to use selection tracking of the table widget itself. It refers to selected table cells.
self.table_view.itemSelectionChanged.connect(self.scene_selection_changed)
If you want to find out which cells was selected, you also should request this information from the table widget, not from the scene.