chore: remove 3.9 tests from kokoro

[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 continuous integration configurations to remove references to Python 3.9 in the Kokoro environment. This change is necessary because Python 3.9 is no longer available in the Kokoro base image, ensuring that our CI pipelines continue to function correctly by transitioning to Python 3.10 for relevant tests.

Highlights

• Kokoro CI Configuration Update: Updated Kokoro presubmit configuration to use "system-3.10" for NOX_SESSION instead of "system-3.9", aligning with the removal of Python 3.9 from the Kokoro base image.
• Python Version Support in Noxfile: Modified noxfile.py to reflect the change, updating SYSTEM_TEST_PYTHON_VERSIONS from "3.9" to "3.10" and removing "unit-3.9" from the default nox.options.sessions.

Changelog

• .kokoro/presubmit/system.cfg
  • Changed the NOX_SESSION environment variable from "system-3.9" to "system-3.10".
• noxfile.py
  • Updated the SYSTEM_TEST_PYTHON_VERSIONS list to include "3.10" instead of "3.9".
  • Removed "unit-3.9" from the nox.options.sessions list.

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[gemini-code-assist]

Code Review

This pull request removes Python 3.9 tests from the Kokoro CI configuration, which is necessary as Python 3.9 has been removed from the Kokoro base image. The changes involve updating the system test session to use Python 3.10 instead of 3.9 in .kokoro/presubmit/system.cfg and noxfile.py. Additionally, the unit-3.9 test session is removed from the default nox.options.sessions list. The changes are consistent and correctly implement the stated goal. The code looks good.