[3.6] bpo-34246: Use no mutable default args in smtplib (GH-8554) (#9112)

[3.6] bpo-34246: Use no mutable default args in smtplib (GH-8554) (#9112)

Some methods of the SMTP class use mutable default arguments. Specially
`send_message` is affected as it mutates one of the args by appending items
to it, which has side effects on further calls..
(cherry picked from commit d5fbe9b1a3d65ceeb9159c5ba999ee966a945f76)

Co-authored-by: Pablo Aguiar <scorphus@gmail.com>