docs: def iiif_image_key(obj):
      """Generate the IIIF image key."""
      if isinstance(obj, ObjectVersion):
          bucket_id = obj.bucket_id
          version_id = obj.version_id
          key = obj.key
      else:
          bucket_id = obj.get('bucket')
          version_id = obj.get('version_id')
          key = obj.get('key')
      return u'{}:{}:{}'.format(
          bucket_id,
          version_id,
          key,
      )