chore: remove 3.9 from kokoro tests

[daniel-sanche]

3.9 has been removed from the kokoro base image, so we should remove the tests from our configs

3.9 is still tested in GitHub Actions

[gemini-code-assist]

Summary of Changes

Hello @daniel-sanche, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the project's continuous integration configuration by removing Python 3.9 from the default Kokoro test suite. This change aligns the testing setup with the updated Kokoro base image, which no longer includes Python 3.9, thereby streamlining the CI process. Python 3.9 test coverage is maintained through existing GitHub Actions workflows.

Highlights

• Kokoro Test Configuration: Removed Python 3.9 from the default Kokoro test sessions in noxfile.py.
• Python 3.9 Support: Confirmed that Python 3.9 is still tested in GitHub Actions, ensuring continued coverage despite its removal from Kokoro.

Changelog

• noxfile.py
  • Removed 'unit-3.9' from the list of default Nox sessions.

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[gemini-code-assist]

Code Review

This pull request removes Python 3.9 from the default Kokoro test sessions by updating nox.options.sessions. The change is correct and achieves the stated goal. I have added one suggestion to improve the maintainability of the session list by generating it programmatically, which would make future updates easier.

[gemini-code-assist]

To improve maintainability, consider generating the list of unit test sessions programmatically instead of maintaining it manually. This would make it easier to update the versions tested in Kokoro in the future by changing them in a single place.

For example:

# Python versions to test in Kokoro CI.
KOKORO_PYTHON_VERSIONS = ["3.10", "3.11", "3.12", "3.13", "3.14"]

nox.options.sessions = [f"unit-{version}" for version in KOKORO_PYTHON_VERSIONS] + [
    "system_emulated",
    "system",
    "mypy",
    "cover",
    "lint",
    "lint_setup_py",
    "blacken",
    "docs",
    "docfx",
    "format",
]

[chalmerlowe]

LGTM