Default python3 on Debian buster:
$ python3
Python 3.7.3 (default, Jan 22 2021, 20:04:44) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import email.mime.text
>>> mt = email.mime.text.MIMEText('Ta mère', 'plain', 'utf-8')
>>> print(mt.as_string())
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: base64

VGEgbcOocmU=

>>> email.encoders.encode_quopri(mt)
>>> print(mt.as_string())
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Transfer-Encoding: quoted-printable

Ta=20m=C3=A8re

So the encoded text looks good now, but there are still duplicate headers.

Old output (python2.7) is identical to what Asheesh Laroia (paulproteus)
reported for python2.5:
---
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Transfer-Encoding: quoted-printable

VGEgbcOocmU=3D
---