UnicodeEncodeError: 'ascii' codec can't encode character

用下面的可以发送中文，题主那段代码用ascii编码中文内容会出错

import smtplib
from email.mime.text import MIMEText
from email.header import Header


from_addr = 'xxx@xxx.xxx'
password = 'xxxx'
to_addr = 'xxx@xx.xxx'
smtp_server = 'xx.xx.xx'

msg = MIMEText('使用python发送邮件测试', 'plain', 'utf-8')

msg['From'] = Header('张三') 
msg['To'] = Header('李四') 
subject = 'Python SMTP 邮件测试'
msg['Subject'] = Header(subject, 'utf-8') 

try:
    smtpobj = smtplib.SMTP_SSL(smtp_server)
    smtpobj.connect(smtp_server, 465)  
    smtpobj.login(from_addr, password)   
    # 发送邮件
    smtpobj.sendmail(from_addr, to_addr, msg.as_string()) 
    print("邮件发送成功")
except smtplib.SMTPException:
    print("无法发送邮件")
finally:
    # 关闭服务器
    smtpobj.quit()