fix(bigquery-magics): Drop support for Python 3.9

[chalmerlowe]

This PR updates `bigquery-magics` to drop support for Python 3.9 and establish Python 3.10 as the minimum supported version. (Note: `setup.py` was already at Python 3.10, so this cleans up the remaining test configs!).

Changes

• Configuration: Updated `noxfile.py` to remove 3.9 mappings from unit and system test extras, and updated supported versions.
• Documentation: Updated `README.rst` and `CONTRIBUTING.rst` to reflect supported Python versions.

Verified successfully with 126 passing tests under Python 3.10!

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

[gemini-code-assist]

Code Review

This pull request updates the supported Python versions by dropping Python 3.9 and adding Python 3.14 across the documentation and Nox configuration. Review feedback points out that Python 3.14 is missing from several lists and link references in the contribution guide. Additionally, there are suggestions to simplify the Nox file by consolidating extras and a reminder to update project metadata in setup.py or pyproject.toml to reflect the new minimum version requirement.

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packages/bigquery-magics/CONTRIBUTING.rst (201)

The list of supported Python versions here is missing Python 3.14, which was added to the supported versions list on line 25 and is present in the noxfile.py configuration. Please add it to this list to maintain consistency.

References

1. When updating the list of supported Python versions in documentation, ensure that all references are updated consistently across the file.

packages/bigquery-magics/CONTRIBUTING.rst (206)

Please add a link reference for Python 3.14 (e.g., .. _Python 3.14: https://docs.python.org/3.14/) to match the updated version list on line 25.

References

1. When updating supported Python versions in reStructuredText documentation, ensure that all version-specific link targets are present and correctly defined to avoid broken references.

packages/bigquery-magics/noxfile.py (63-65)

With the removal of Python 3.9 support and 3.10 becoming the minimum, the bqstorage extra (and bigframes in system tests) no longer needs to be version-specific. Consider moving these to the common UNIT_TEST_EXTRAS and SYSTEM_TEST_EXTRAS lists. Additionally, ensure that the project's metadata in setup.py or pyproject.toml is updated to reflect Python 3.10 in python_requires and Trove classifiers. Since dropping a Python version is a breaking change, ensure this is handled as a minor version bump rather than a patch.

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.
2. When a release introduces breaking changes in environment requirements, such as dropping support for specific Python versions, prefer a minor version bump over a patch version bump.