Constraining list/array types in the schema

[ml-evs]

I don't have time for a full explanation (perhaps @ltalirz/@dwinston can elaborate), but we still have issues in constraining e.g. the lengths of lists in a way that is compatible with both pydantic and FastAPI.

Currently we have conlist which has just been released in pydantic, but has no effect on the generated API pydantic/pydantic/issues/604. It is not clear whether the types inside the conlist can be constrained, as of yet.

We also need a way to constrain lists of lists, e.g. ensuring lattice vectors are always 3x3, and if possible, constraining e.g. cartesian_site_positions to have length nsites, in order to fully validate a JSON response against the spec.

Some additional discussion can be found in the comments to #45.

[ltalirz]

@ml-evs @dwinston fastapi 0.31.0 has just been released and should be compatible with pydantic 0.29

This (hopefully) brings us a working conlist implementation.