title: Filtering on a model's calculated method using django_filter
body: <p>I have a Django model with a calculated field on it.  As an example:</p>
  
  <pre><code>class MyModel(models.Model):
      number = models.IntegerField(default=3)
  
      @property
      def incremented(self):
          return self.number + 1
  </code></pre>
  
  <p>I'd like to filter on this property, so I tried the following:</p>
  
  <pre><code>class ModelFilter(django_filter.FilterSet):
      class Meta:
          model = MyModel
          fields = ('incremented',)
  </code></pre>
  
  <p>That did not work, and neither did this:</p>
  
  <pre><code>class ModelFilter(django_filter.FilterSet):
      incremented = django_filter.NumberFilter(lookup_type='exact')
      class Meta:
          model = MyModel
          fields = ('incremented',)
  </code></pre>
  
  <p>What am I doing wrong here?</p>