Returns a GEOS Polygon from bounding box values.

def to_python(self, value):
        """Returns a GEOS Polygon from bounding box values."""
        value = super(BoundingBoxField, self).to_python(value)
        try:
            bbox = gdal.OGRGeometry.from_bbox(value).geos
        except (ValueError, AttributeError):
            return []
        bbox.srid = self.srid
        return bbox