According to rfc2046, line breaks in MIME are CRLF.
However python just
uses LF like in the following example:

from email.MIMEMultipart import MIMEMultipart
from email.MIMEText import MIMEText

msg = MIMEMultipart()
msg['Subject'] = 'Our family reunion'
msg['From'] = '...@a.b'
msg['To'] = '...@x.y'
msg.epilogue = ''

msg.attach(MIMEText('aaaaaaaaaaaaaaaaaaaaaaaa'))

print `msg.as_string()`

gives:
'Content-Type: multipart/mixed;
boundary="===============1018679223=="\nMIME-Version:
1.0\nSubject: Our
family reunion\nFrom: a...@a.b\nTo:
c...@x.y\n\n--===============1018679223==\nContent-Type:
text/plain;
charset="us-ascii"\nMIME-Version:
1.0\nContent-Transfer-Encoding:
7bit\n\naaaaaaaaaaaaaaaaaaaaaaaa\n--===============1018679223==--\n'


Found in version 2.3 and 2.4