Drop support for Python 3.9 (track post 3.9 changes)

[garth-wells]

Drop support for Python 3.9 when it reaches end-of-life in October 2025 (https://devguide.python.org/versions/).

Task/changes:

• Undo Deactivate ruff 3.9 deprecation #3757
• Remove typing.Optional #3755
• Simplify type hint unions (https://peps.python.org/pep-0604/)