The following produces a non-conformant message, since the us-ascii
charset is strictly 7bit:

>>> import email.message
>>> m = email.message.Message()
>>> m.set_payload("""A few lines
... of 8-bit text
...
... One high bit character: ².
... """, 'us-ascii')
>>> print m.as_string()
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 8bit

A few lines
of 8-bit text

One high bit character: ².

>>>

In Python2 the fix would be to use charset unknown-8bit instead of us-ascii.

In Python3 this actually puts unicode in the message body.  There we should default to utf-8, but this requires a more extensive change than the Python2 change, and probably should not be backported.

Once this is fixed in Python3 the utf-8 default check can be removed from MIMEText (issue 14380).