Bump the classifier from Alpha to Beta

[Julian]

Once #28 is closed and we've added referencing.to_cached_resource, we'll change referencing's classifiers to mark it in Beta.

It will stay in this state until some or all of:

• it has support beyond the JSON Schema specifications (i.e. OpenAPI)
• another library beyond jsonschema uses it
• jsonschema has used it for around a year