Give created QGraphicsScenes a parent

Ensure the scene is deleted at the same time as the widget.
Fix an runtime error at process exit when running tests with PyQt 5.14

Author: ales-erjavec

Orange/widgets/visualize/owboxplot.py

@@ -263,7 +263,7 @@ def __init__(self):
             stateWhenDisabled=False)
 
         gui.vBox(self.mainArea, addSpace=True)
-        self.box_scene = QGraphicsScene()
+        self.box_scene = QGraphicsScene(self)
         self.box_scene.selectionChanged.connect(self.commit)
         self.box_view = QGraphicsView(self.box_scene)
         self.box_view.setRenderHints(QPainter.Antialiasing |

Orange/widgets/visualize/owvenndiagram.py

@@ -89,7 +89,7 @@ def __init__(self):
         self.itemsets = OrderedDict()
 
         # Main area view
-        self.scene = QGraphicsScene()
+        self.scene = QGraphicsScene(self)
         self.view = QGraphicsView(self.scene)
         self.view.setRenderHint(QPainter.Antialiasing)
         self.view.setBackgroundRole(QPalette.Window)