fix(db-dtypes): Drop support for Python <= 3.9

[chalmerlowe]

This PR updates `db-dtypes` to establish Python 3.10 as the minimum supported version, dropping support for Python 3.7, 3.8, and 3.9.

Changes

• Configuration: Updated setup.py metadata and python_requires to >=3.10.
• Nox: Updated noxfile.py sessions and deleted constraints-3.9.txt.
• Documentation: Updated README.rst and CONTRIBUTING.rst (and synced to docs/README.rst!).
• Removed extract_runtime_version and its helper file from db_dtypes
• Updated the tests to mock sys.version_info correctly.

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

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9, updating documentation, CI configurations, and package metadata to reflect a new minimum requirement of Python 3.10. The runtime version check has been simplified by removing the _versions_helpers module and using sys.version_info directly. Feedback suggests moving the sys import to the top level for idiomatic consistency and ensuring that versioning follows standard policies when dropping support.

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packages/db-dtypes/db_dtypes/init.py (33)

It is more idiomatic to import sys at the top level of the module rather than inside a function, especially for a lightweight standard library module. This also avoids re-importing it every time _check_python_version is called.

import sys

packages/db-dtypes/db_dtypes/init.py (343-345)

The local import of sys can be removed if it is moved to the top level. Additionally, when updating the minimum supported Python version in runtime checks (e.g., sys.version_info), ensure that the project's metadata in setup.py or pyproject.toml is updated to reflect the same minimum version. If this change drops support for a Python version, prefer a minor version bump over a patch version bump.

    if sys.version_info < (3, 10):

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