bpo-41425: Make tkinter doc example runnable (GH-21706)

Co-authored-by: Ankit Chandawala <achandaw@amazon.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Doc/library/tkinter.rst

@@ -541,31 +541,35 @@ the variable, with no further intervention on your part.
 
 For example::
 
-   class App(Frame):
-       def __init__(self, master=None):
+   import tkinter as tk
+
+   class App(tk.Frame):
+       def __init__(self, master):
            super().__init__(master)
            self.pack()
 
-           self.entrythingy = Entry()
+           self.entrythingy = tk.Entry()
            self.entrythingy.pack()
 
-           # here is the application variable
-           self.contents = StringVar()
-           # set it to some value
+           # Create the application variable.
+           self.contents = tk.StringVar()
+           # Set it to some value.
            self.contents.set("this is a variable")
-           # tell the entry widget to watch this variable
+           # Tell the entry widget to watch this variable.
            self.entrythingy["textvariable"] = self.contents
 
-           # and here we get a callback when the user hits return.
-           # we will have the program print out the value of the
-           # application variable when the user hits return
+           # Define a callback for when the user hits return.
+           # It prints the current value of the variable.
            self.entrythingy.bind('<Key-Return>',
-                                 self.print_contents)
+                                self.print_contents)
 
        def print_contents(self, event):
-           print("hi. contents of entry is now ---->",
+           print("Hi. The current entry content is:",
                  self.contents.get())
 
+   root = tk.Tk()
+   myapp = App(root)
+   myapp.mainloop()
 
 The Window Manager
 ^^^^^^^^^^^^^^^^^^
@@ -860,4 +864,4 @@ use raw reads or ``os.read(file.fileno(), maxbytecount)``.
           WRITABLE
           EXCEPTION
 
-   Constants used in the *mask* arguments.
+   Constants used in the *mask* arguments.

Misc/NEWS.d/next/Library/2020-08-03-01-59-48.bpo-41425.KJo6zF.rst

@@ -0,0 +1 @@
+Make tkinter doc example runnable.