Upgrade to Python 3.10+ (3.9 broken anyway)

[athewsey]

agent-evaluation currently specifies Python 3.9+, as per:

• setup.py install_requires
• setup.py classifiers
• github CI
• installation docs
• sample streamlit app readme

...but:

1. Python 3.9 will stop receiving security updates in 2025-10, and
2. Support is already broken by various uses of | in type expressions - which was only introduced in Python 3.10 in PEP604. When I try to run pytest on a 3.9 environment, I see a slew of TypeError: unsupported operand type(s) for |: 'type' and 'NoneType' errors.

...So I probably suggest to just bump the listed dependency to 3.10+?