Implement tests without calling the real API

[licht1stein]

It would be a good practice to switch to tests that use mock API instead of actual production API.

[phistrom]

Does Basecamp 3 have a mock/test API? Or are you saying we should write something that responds as if it were the real API?

[licht1stein]

We'll mock it in the tests. I'll play around a bit to show you, after the poetry switch. Cause poetry will let us have separate dev dependencies.