title: How can I load 'null' from .yml as None (class str), not (class NoneType)?
body: <p>I have yaml that looks like this:</p>
  
  <pre><code>  SomeRecord:
      type: array
      items:
        type:
          - string
          - number
          - null
  </code></pre>
  
  <p>I try PyYAML and ruamel.yaml they both convert <code>'null'</code> to <code>None</code> (class NoneType).
  Is there an easy way to change this behavior?</p>