Create configuration variable for python version in Github workflows

[dougiesquire]

The Python version to build against is being used in a number of places in .github/workflows/CI.yml and .github/workflows/CD.yml. I tried to use a configuration variable in #403 to define this in a single location, but this wasn't working at the time, so instead I used a workflow-level environment variable in each workflow. These should be replaced with a Repository or Environment configuration variable.

ping @CodeGat

[dougiesquire]

E.g. a "release" github Environment that we could also use for the PyPI trusted publishing

[CodeGat]

I've done a lot of Environment stuff so I'll be able to help out :)