chore: remove references to Python 3.8 and 3.9

[chalmerlowe]

This PR updates pandas-gbq 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 and remove dead skip logic for 3.9.
• Documentation: Updated CONTRIBUTING.rst to reflect the new supported Python versions and update Nox examples.
• Runtime Check: Updated the runtime version check in pandas_gbq/__init__.py to warn for versions older than 3.10.

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

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9 across the codebase, updating the contribution guidelines, CI configurations in noxfile.py, and runtime warning messages to reflect a new minimum requirement of Python 3.10. Feedback highlights that setup.py or pyproject.toml must be updated to reflect the new python_requires, and a minor version bump is required for this breaking change. Additionally, missing documentation link targets for Python 3.13 and 3.14 should be added to avoid broken references.

[gemini-code-assist]

The links for Python 3.13 and 3.14 are missing from the bottom of the file. Since these versions are listed as supported in the list above, their corresponding link targets should be added to avoid broken references in the documentation.

Suggested change

	.. _Python 3.10: https://docs.python.org/3.10/
	.. _Python 3.11: https://docs.python.org/3.11/
	.. _Python 3.12: https://docs.python.org/3.12/
	.. _Python 3.10: https://docs.python.org/3.10/
	.. _Python 3.11: https://docs.python.org/3.11/
	.. _Python 3.12: https://docs.python.org/3.12/
	.. _Python 3.13: https://docs.python.org/3.13/
	.. _Python 3.14: https://docs.python.org/3.14/

[chalmerlowe]

@gemini-code-assist:
Thanks for doing these checks!

In this case, please note: these links are not missing. They might be outside the diff's context window for the change OR something. But if you check, the lines you ask for are already there.

[gemini-code-assist]

Apologies for the confusion. You are correct; upon re-examining the file content, the link definitions for Python 3.13 and 3.14 are indeed present in the file. My previous comment was based on an incomplete view of the file context. Thank you for clarifying.