Create a property descriptor around the :class:`_PropertyMixin` helpers.

def _scalar_property(fieldname):
    """Create a property descriptor around the :class:`_PropertyMixin` helpers.
    """

    def _getter(self):
        """Scalar property getter."""
        return self._properties.get(fieldname)

    def _setter(self, value):
        """Scalar property setter."""
        self._patch_property(fieldname, value)

    return property(_getter, _setter)