Update codebase to cooperate with Marshmallow 3.0.0

[Marshmallow 3.0.0](https://marshmallow.readthedocs.io/en/stable/changelog.html) introduces some breaking changes that...er...break our code.

The issue seems to occur when [dumping the schema.](https://github.com/brighthive/authserver/blob/1ee40c2c3f488d2bcc0eba0373855929c6012d0b/authserver/api/client.py#L33)

Temporarily, we have the Pipfile pinned to `2.19.5`.