Bring geddit up to speed with modern CI templates

Bring geddit up to speed with our common CI pipeline as described in https://guidebook.devops.uis.cam.ac.uk/en/latest/howtos/add-common-pipeline-to-python/.

If easy enough, consider adding pre-commit, etc so that it is in line with https://guidebook.devops.uis.cam.ac.uk/en/latest/tutorials/creating-a-python-package/.