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

[miss-islington]

Co-authored-by: Ankit Chandawala [email protected]
Co-authored-by: Terry Jan Reedy [email protected]
(cherry picked from commit c36dbac)

Co-authored-by: Ankit Chandawala [email protected]

https://bugs.python.org/issue41425

[the-knights-who-say-ni]

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[miss-islington]

@nerandell and @terryjreedy: Status check is done, and it's a success ✅ .