Create an RSTTree instance from a string content an *.rs3 file.

def fromstring(cls, rs3_string):
        """Create an RSTTree instance from a string content an *.rs3 file."""
        temp = tempfile.NamedTemporaryFile(delete=False)
        temp.write(rs3_string)
        temp.close()
        rst_tree = cls(rs3_file=temp.name)
        os.unlink(temp.name)
        return rst_tree