Remove support for Python 3.9 (EOL)

[davidhassell]

Python 3.9 is nearly EOL (2025-10), and is no longer supported by the latest versions of various dependencies (e.g. numpy, Dask.).

Let's set the new minimum to Python 3.10.

[sadielbartholomew]

Very happy to drop 3.9 - then we can go in and add match-case statement anywhere plausible 🎉