More type checking fixes after the merge.

CURA-2917

cura/QualityManager.py

@@ -18,7 +18,7 @@ class QualityManager:
             QualityManager.__instance = cls()
         return QualityManager.__instance
 
-    __instance = None
+    __instance = None   # type: 'QualityManager'
 
     ##  Find a quality by name for a specific machine definition and materials.
     #

cura/Settings/ContainerManager.py

@@ -694,7 +694,7 @@ class ContainerManager(QObject):
             ContainerManager.__instance = cls()
         return ContainerManager.__instance
 
-    __instance = None
+    __instance = None   # type: 'ContainerManager'
 
     # Factory function, used by QML
     @staticmethod

cura/Settings/ProfilesModel.py

@@ -38,7 +38,7 @@ class ProfilesModel(InstanceContainersModel):
             ProfilesModel.__instance = cls()
         return ProfilesModel.__instance
 
-    __instance = None
+    __instance = None   # type: 'ProfilesModel'
 
     ##  Fetch the list of containers to display.
     #