Emails sent by the API are not received by free.fr email addresses

For instance registration or email change emails. See the UI issue @desnoes has created: https://github.com/c2corg/v6_ui/issues/1763

It is maybe related to this function https://github.com/c2corg/v6_api/blob/master/c2corg_api/emails/email_service.py#L53-L72
We used to send emails with ``transfer_encoding`` defaulting to ``quoted-printable`` but had to use ``base64`` instead (using an explicit ``Attachment``) because of encoding problems in messages received by hotmail addresses.