Sample code for: Python 3.15 Preview: Sentinel Values

[lpozo]

Where to put new files:

• New files should go into a top-level subfolder, named after the article slug. For example: my-awesome-article

How to merge your changes:

1. Make sure the CI code style tests all pass (+ run the automatic code formatter if necessary).
2. Find an RP Team member on Slack and ask them to review & approve your PR.
3. Once the PR has one positive ("approved") review, GitHub lets you merge the PR.
4. 🎉

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[realpython-bot]

Final QA: all 34 scripts run as expected on Python 3.15.0rc2, and the output matches the tutorial. README title and link match the slug. Fixed example_006.py to print the enum's repr(), as shown in the tutorial. ruff 0.14.1 format and check pass.