title: None in boost.python
body: <p>I am trying to translate the following code </p>
  
  <pre><code>d = {}
  d[0] = None
  </code></pre>
  
  <p>into C++ with boost.python</p>
  
  <pre><code>boost::python::dict d;
  d[0] = ?None
  </code></pre>
  
  <p>How can I get a None object in boost.python?</p>