title: Serializing data with marshmallow if data key is unknown
body: <p>I'm trying to serialise data where the data key is not known beforehand. The data would look something like this:</p>
  
  <pre><code>{
      "unknown key": ["list", "of", "random", "strings"]
  }
  </code></pre>
  
  <p>Is this something that's possible with marshmallow? I can't seem to find anything in the docs about it.</p>