Also upgrade from openapi version 3.0.3 to 3.1.0

- nullables are replaced by the "null" type.
- application/octet-stream content schema don't require specifying a schema

Which will allow adding metadata to resultsets.

disabled by default, marked as experimental

OpenAPI spec now always shows the login/logout routes

JSON routes do not need it but added for consistency

The login route is now disabled by default.
It was previously enabled by default.
It is why it is a breaking change.

Mount on "/" when we are on an older version of cubicweb missing the receives-base-url-path OR when the receivese-base-url-path option is set to False. This option has been added in version 4.2.

Otherwise we are serving our routes on the full base url.

Closes #34.