Drop support for python3.9 #832

Github has updated its version of Ubuntu to 26.04 which doesn't support python3.9 which means PRs fail to validate with this:

Error: The version '3.9' with architecture 'x64' was not found for Ubuntu 26.04.
The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

The last version of ubuntu to support python3.9 was Ubuntu 22.04