Test against development versions of upstream repositories

[znicholls]

Set up CI that runs against development versions of our upstream repositories. This would help both us and our upstream repositories with maintenance and greatly reduce the chance of surprise breaks as new versions of upstream repositories are released.

There is good discussion and links in hgrecco/pint#1631 for when we come to actually implementing this. Copying the key ideas from xarray might be the best option.

Arose from #275