Revisit MacOS CI support

[AlexanderWells-diamond]

PR #158 was done to fix GitHub changing the macos-latest tag to now point to an arm64 image, rather than a x64 one (it also bumped from MacOS-12 to MacOS-14 at the same time).

We currently cannot use arm64 as there is no Python3.7 support available. Hence we currently use macos-13 in all CI steps.

As Python3.7 is now well out of end-of-life, we should look at dropping support for it. This would enable us to begin using the arm64 images. We may also want to look at running on both types of MacOS runner (if such a configuration is possible)

[AlexanderWells-diamond]

This issue will also be affected by cothread issue #68, which also won't work on arm64 platforms.