Force \r\n line endings when performing PGP cryptography.

Force \r\n line endings when performing PGP cryptography.

From RFC 3156, section 5 (OpenPGP signed data):

  When the OpenPGP digital signature is generated:

   (1)   The data to be signed MUST first be converted to its content-
         type specific canonical form.  For text/plain, this means
         conversion to an appropriate character set and conversion of
         line endings to the canonical <CR><LF> sequence.

This will be easier with Python 3.3's policies:

  email.policy.SMTP

    Output serialized from a message will conform to the email and
    SMTP RFCs. The only changed attribute is linesep, which is set to
    \r\n.