Test API stability

[jonthegeek]

I'm setting up tests to use mocks, so we don't actually hit the API. This feels important for this package, since a lot of the calls can cost actual money (although we'd have to call a lot of them quite a lot for it to add up, but still).

However, I'd also like to check that the API itself hasn't changed.

Set up... something... to actually check the API. I'm thinking probably a special github action? We'll have to look into what makes sense. Just run it periodically, independent of changes to the package. Come to think of it, maybe this should happen via a lambda...