Updated billboard-walkthrough.md to use Python 3 environment setup

[TAMdrew]

Pull Request Title:

Update billboard-walkthrough.md for Python 3 environment setup

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Pull Request Description:

This PR updates the billboard-walkthrough.md file to reflect the use of Python 3 for environment setup, replacing references to Python 2.7. These changes ensure compatibility with modern Python versions, resolving deprecation warnings, and improving future maintainability.

Key changes include:

• Updated the environment setup instructions to use python3 -m venv for creating virtual environments.
• Ensured that the instructions are clear and consistent with the requirements for Python 3.
• Removed outdated references to Python 2.7 in the setup process.

Testing and Validation:

• The updated environment setup has been tested locally using Python 3.10.
• All steps for creating the virtual environment, installing dependencies, and running the script have been successfully validated.
• No unit tests were required for this documentation update, but the changes were manually verified.

Additional Updates:

• No changes to the main codebase were necessary, as this PR only affects documentation.
• The relevant sections now adhere to the Google Style Guide.

Licensing:

• All modified files retain the existing Apache 2.0 license with up-to-date copyright attribution to Google LLC.

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This PR resolves compatibility issues with the deprecated Python 2.7 environment setup, ensuring a smoother onboarding experience for users.

[TAMdrew]

@agold-rh Would you be able to trigger the build with /gcbrun? Thanks!

[agold-rh]

/gcbrun