Set string value of *name* property to *value*.

def _set_element_text(self, prop_name, value):
        """Set string value of *name* property to *value*."""
        if not is_string(value):
            value = str(value)

        if len(value) > 255:
            tmpl = (
                "exceeded 255 char limit for property, got:\n\n'%s'"
            )
            raise ValueError(tmpl % value)
        element = self._get_or_add(prop_name)
        element.text = value