Scheduled tests for released MUSE versions

For the Open University course I've instructed users to specifically install version 1.2.1, as this is what all the tutorials were written with, and future versions may give different results or not work with these scenarios. (v1.2.1 is currently a release candidate, but will be promoted to a proper release by the time the couse comes online).

One issue that I could anticipate is that, since we're not pinning dependency versions, running pip install a few months from now might pull in a different set of dependency versions, which may break things. We've certainly had this issue before with xarray, pandas and numpy.

Therefore, I think it would be a good idea to run scheduled tests for this version for as long as this version of the course is online. If necessary, we can then create a [post-release](https://peps.python.org/pep-0440/#post-releases) (e.g. 1.2.1.post1) to fix any problems.

I can see there's something already in place to run regression tests weekly, which we can build off, although this needs updating as it's still testing with Python 3.8