Instead of encodestring consider using b64encode. Later does not add \n characters. e.g.
In [11]: auth = b'username@domain.com:passWORD'
In [12]: base64.encodestring(auth)
Out[12]: b'dXNlcm5hbWVAZG9tYWluLmNvbTpwYXNzV09SRA==\n'
In [13]: base64.b64encode(auth)
Out[13]: b'dXNlcm5hbWVAZG9tYWluLmNvbTpwYXNzV09SRA=='
It produces identical encoded string except the \n