Drop support for Python 3.9

[msto]

Summary

This issue is forward-looking to the end of Python 3.9 support in October 2025.

Python 3.10 Features

Dropping support for Python 3.9 will give us access to the following Python 3.10 features:

1. Structural Pattern Matching (match-case statements)
2. Parenthesized context managers (e.g. with (in_path.open() as fin, out_path.open() as fout))
3. Length-Checking in zip() (i.e. zip(..., strict=True))
4. Writing Union Types as X | Y
5. typing.TypeGuard and typing.TypeAlias (both available to Python 3.9 through typing_extensions)