Investigate ways of removing circular dependencies

[mesozoic]

There are a number of type annotations that create circular dependencies in the library, which triggered #411. That particular issue is being resolved upstream in Pydantic, but it's conceivable that this could cause other issues with other libraries or toolchains in the future.

The way we've set up the class hierarchy (where all contained classes have a reference to their parents, so everything has access to an Api instance) it's not clear how we would remove the circular type annotations. But I'll leave this here in case anyone has an idea.