Drop YAML support

Follow-up of !241 (merged)

Arkindex can output the API schema as JSON directly rather than YAML, which can be faster and allows to use Python's json module instead of depending on PyYAML. The API client will try to get JSON by default, but parses the schema as YAML anyway because users may specify a different API schema URL that does not use JSON.

We could try dropping support for YAML entirely, adding ?format=json or Accept: application/json manually when requesting the schema to try to get JSON.