Update marshmallow to 3.0+

marshmallow, which Tildes uses for schema validation and some similar applications, has now released its 3.0 version, after a fairly long RC process. This will require some significant updates to the code, so I've pinned it to 2.x for now but should try to do these updates before too long, since 3.0 seems to have quite a few benefits.

Here's the docs on upgrading: https://marshmallow.readthedocs.io/en/stable/upgrading.html