gTTS文字转语音如何保存为mp3格式

from gtts import gTTS   
#gtts is imported
audio = 'Hello , I am Tina, your digital assistant'
def talkToMe(audio, lgg = 'en'):
    #print(audio)
    tts = gTTS(text = audio, lang = lgg)
    tts.save('audio.mp3') #doesn't work
    return None
talkToMe(audio , lgg ='en')