随即产生n个字节的字符串，可以作为随机加密key使用~
>>> index = os.urandom(2) >>> print index 墿 >>> index = os.urandom(2) >>> print index 氡 >>> index = os.urandom(2) >>> print index Ε >>> index = os.urandom(2) >>> print index E‘ >>> index = os.urandom(2) >>> print index €H >>> index = os.urandom(2) >>> print index I? >>> index = os.urandom(2) >>> print index