Issue #3965: Allow repeated calls to turtle.Screen, by making it a
true singleton object.

Reviewed by Gregor Lingl.
diff --git a/Demo/turtle/turtleDemo.py b/Demo/turtle/turtleDemo.py
index 245eaa7..5e744ef 100644
--- a/Demo/turtle/turtleDemo.py
+++ b/Demo/turtle/turtleDemo.py
@@ -94,8 +94,8 @@
         left_frame.pack(side=LEFT, fill=BOTH, expand=0)
         self.graph_frame = g_frame = Frame(root)
 
-        turtle.Screen._root = g_frame
-        turtle.Screen._canvas = turtle.ScrolledCanvas(g_frame, 800, 600, 1000, 800)
+        turtle._Screen._root = g_frame
+        turtle._Screen._canvas = turtle.ScrolledCanvas(g_frame, 800, 600, 1000, 800)
         #xturtle.Screen._canvas.pack(expand=1, fill="both")
         self.screen = _s_ = turtle.Screen()
         self.scanvas = _s_._canvas