Issue #21823: Catch turtle.Terminator exceptions in turtledemo.

Add note to demohelp.txt about doing so.

Lib/turtledemo/clock.py

@@ -11,6 +11,7 @@ and time
   ------------------------------------
 """
 from turtle import *
+from turtle import Terminator  # not in __all__
 from datetime import datetime
 
 mode("logo")
@@ -102,7 +103,8 @@ def tick():
     sekunde = t.second + t.microsecond*0.000001
     minute = t.minute + sekunde/60.0
     stunde = t.hour + minute/60.0
-    tracer(False)
+    try:
+        tracer(False)  # Terminator can occur here
         writer.clear()
         writer.home()
         writer.forward(65)
@@ -113,11 +115,13 @@ def tick():
                      align="center", font=("Courier", 14, "bold"))
         writer.forward(85)
         tracer(True)
-    second_hand.setheading(6*sekunde)
+        second_hand.setheading(6*sekunde)  # or here
         minute_hand.setheading(6*minute)
         hour_hand.setheading(30*stunde)
         tracer(True)
         ontimer(tick, 100)
+    except Terminator:
+        pass  # turtledemo user pressed STOP
 
 def main():
     tracer(False)

Lib/turtledemo/demohelp.txt

@@ -62,9 +62,13 @@
        in the Label below the source code window (when execution
        has finished.)
 
-       !! For programs, which are EVENT DRIVEN, main must return
-       !! the string "EVENTLOOP". This informs the viewer, that the
-       !! script is still running and must be stopped by the user!
-
+       If the demo is EVENT DRIVEN, main must return the string
+       "EVENTLOOP". This informs the demo viewer that the script is
+       still running and must be stopped by the user!
 
+       If an "EVENTLOOP" demo runs by itself, as with clock, which uses
+       ontimer, or minimal_hanoi, which loops by recursion, then the
+       code should catch the turtle.Terminator exception that will be
+       raised when the user presses the STOP button.  (Paint is not such
+       a demo; it only acts in response to mouse clicks and movements.)
 

Lib/turtledemo/minimal_hanoi.py

@@ -18,6 +18,7 @@ stretched to rectangles by shapesize()
  ---------------------------------------
 """
 from turtle import *
+from turtle import Terminator  # not in __all__
 
 class Disc(Turtle):
     def __init__(self, n):
@@ -50,9 +51,12 @@ def hanoi(n, from_, with_, to_):
 def play():
     onkey(None,"space")
     clear()
+    try:
         hanoi(6, t1, t2, t3)
         write("press STOP button to exit",
               align="center", font=("Courier", 16, "bold"))
+    except Terminator:
+        pass  # turtledemo user pressed STOP
 
 def main():
     global t1, t2, t3