title: Update parent dict in subclass
body: <p>I have classes like this:</p>
  
  <pre><code>class Parent(object):
      _defaults = {
          'key': 'value',
          'key2': 'value',
      }
  
  class Child(Parent):
      _defaults = {
          'key2': 'value2',
          'key3': 'value2',
      }
  </code></pre>
  
  <p>When I create the child class, I want <code>_defaults</code> to be set to the parent class <code>_defaults</code> and then updated with the child <code>_defaults</code>. So the actual resolved class should look like this:</p>
  
  <pre><code>class Child(object):
      _defaults = {
          'key': 'value',
          'key2': 'value2',
          'key3': 'value2',
      }
  </code></pre>
  
  <p>Is this possible with a <code>__metaclass__</code> constructor or something?</p>