fix(testutils): Drop support for Python 3.9

[chalmerlowe]

This PR updates `google-cloud-testutils` 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 3.9.
• Constraints: Transferred lower bounds to `constraints-3.10.txt` and dropped `constraints-3.9.txt`.
• Cleanup: Removed obsolete `tests/test_version_warnings.py`.
• Expert Improvements: Modernized `mypy.ini` to 3.10 and added `# version-scanner: ignore` pragmas to test fixtures.

Verified successfully with 35 passing unit tests under Python 3.10!

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

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9, updating the minimum version to 3.10 across configuration files including mypy.ini, noxfile.py, and setup.py. It also migrates the constraints file to Python 3.10 and adds version-scanner ignore comments to internal test resources. Feedback suggests updating the google-auth constraint to a version compatible with Python 3.10 and adding Trove classifiers for Python 3.13 and 3.14 in setup.py to maintain consistency with the supported versions defined in the CI configuration.

[gemini-code-assist]

The python_requires update to >=3.10 is correct. However, the classifiers list (lines 68-70) should also be updated to include Programming Language :: Python :: 3.13 and Programming Language :: Python :: 3.14 to maintain consistency with the versions specified in noxfile.py and ensure metadata reflects the supported versions.

References

1. When updating the minimum supported Python version in runtime checks, ensure that the project's metadata in 'setup.py' or 'pyproject.toml' is also updated to reflect the same minimum version in 'python_requires' and Trove classifiers.
2. Python 3.14 is a valid Python version and should be supported in configurations.

[parthea]

nit: Can we update this rather than ignore it?