fix(bigframes): Drop support for Python <= 3.9

[chalmerlowe]

Note

This is part of a larger effort across the monorepo to ensure that all packages are focused on currently supported Python runtimes. This package was already heavily focused on 3.10 support so changes should be fairly minimal.

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

Changes

• Configuration: Updated noxfile.py to remove references to Python 3.9 from the supported versions list.
• Documentation: Updated CONTRIBUTING.rst to use Python 3.10 in sample commands and removed stale Python 2 compatibility notes.
• Cleanup: Removed dead code in noxfile.py that was skipping Python 3.7, 3.8, and 3.9.
  Removed dead code in tests/unit/test_pandas.py checking for Python < 3.10.

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

[gemini-code-assist]

Code Review

This pull request removes support for Python 3.9 and older versions across the repository. Key changes include updating the contributing documentation, removing Python 3.9 from the Nox configuration, and deleting redundant version-based test skips in the unit and system test suites. Feedback was provided to ensure that the project's metadata in configuration files like setup.py or pyproject.toml is also updated to reflect the new minimum Python version requirement.

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packages/bigframes/noxfile.py (63)

When updating the minimum supported Python version, 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.

References

1. When updating the minimum supported Python version in runtime checks (e.g., 'sys.version_info' 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.