Remove support for Python 3.9 (EOL)

Python 3.9 is EOL ([2025-10](https://devguide.python.org/versions/)), 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.

This issues mirrors https://github.com/NCAS-CMS/cfdm/issues/353