如何将电子邮件正文设置为包含多字节字符的字符串?

当我试图修改已解析电子邮件的正文时,结果消息的编码是错误的。这是一个最小的例子:

raw_message = '''From: user@example.org
To: john@example.com
Subject: Happy new year

Please submit your 50000 word paper tomorrow.
'''

import email
from email import policy

msg = email.message_from_string(raw_message, policy=policy.default)
msg.set_content('天地玄黃宇宙洪荒')
print(msg)

这张照片:

From: user@example.org
To: john@example.com
Subject: Happy new year
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
MIME-Version: 1.0

5aSp5Zyw546E6buD5a6H5a6Z5rSq6I2SCg==

编码错误,无法显示汉字。如何将电子邮件的内容设置为包含多字节字符(如CJK字符)的字符串?

Python版本:Python 3.6.8