End Support of Python 3.9

[bmos]

I'd suggest we remove support for Python 3.9 in December (to avoid negatively impacting Nov elections).

Background

Python 3.9 was first released 2020-10-05. It reaches official end-of-support 2025-10.

Downsides

• Some users may still be running Python 3.9

Upsides

Python Features

• No longer need to use typing.Union, can use | operator between types
• No longer need to use typing.Optional, can use | None
• Can use structural pattern matching with match/case syntax
• More

Dependencies

• Upgrade civis
• Upgrade joblib
• Upgrade myst-parser (docs requirement)
• Upgrade Sphinx (docs requirement)

[bmos]

Shouldn't have the bug label, oops