fix(google-auth): Drop support for Python 3.8 and 3.9

[chalmerlowe]

This PR updates google-auth to establish Python 3.10 as the minimum supported version, dropping support for Python 3.9 and below.

Changes

• Configuration: Updated setup.py and noxfile.py to require Python 3.10+ and remove references to Python 3.7, 3.8, and 3.9.
• Documentation: Updated README.rst and CONTRIBUTING.rst to state that Python 3.10 is the minimum.
• Cleanup: Removed EOL warning code from source and tests.
• Constraints: Transferred lower bounds to constraints-3.10.txt and dropped 3.8/3.9 files.

Note

During local testing with Docker, 7 async tests failed on Python 3.10 inside tests_async/transport/test_aiohttp_requests.py (though all tests passed successfully on 3.11 - 3.14). We are pushing to see if these failures replicate on GitHub Actions or if they are just a local environment quirk.

Fixes internal issue: http://b/482126936 🦕

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.8 and 3.9 across the library, updating documentation, CI configurations in noxfile.py, and package metadata in setup.py. It also removes legacy deprecation warnings and logic related to end-of-life Python versions (3.7, 3.8, and 3.9). The reviewer identified that Python 3.13 and 3.14 classifiers are missing from setup.py and should be added to align with the supported versions mentioned in other configuration files.

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packages/google-auth/setup.py (120-121)

The Python 3.13 and 3.14 classifiers are missing from setup.py. Since these versions are officially supported and tested (as indicated in noxfile.py and CONTRIBUTING.rst), they should be included in the classifiers list to ensure proper indexing on PyPI.

        "Programming Language :: Python :: 3.13",
        "Programming Language :: Python :: 3.14",

References

1. Verify that suggested additions to a configuration list (such as Python versions in a test matrix) are not already present in the file before recommending their inclusion to avoid false positive suggestions.