[FIX] ir_mail_server: Use IDNA for smtp_user

There is a user whose email login is "joe@examplé.com", notice the
latin "é" in the domain. This domain is a valid IDNA-2008 domain but
the smtplib of python is incompatible with such domain and raises a
UnicodeError because the character is not ascii.

Note the removed comment about bytestring is a leftover of a dark
python2 age and is no more valid.

Closes #61972

closes odoo/odoo#61994

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>