fix: allow EmailMultiAlternatives in outbox

[alexei]

According to the Django docs on sending email the outbox attribute is:

a list with an EmailMessage instance for each message that would be sent.

When you send single-message emails, they are direct instances of EmailMessage. When the emails are multipart, they are instances of EmailMultiAlternatives. While EmailMultiAlternatives inherits from EmailMessage, declaring outbox as list[EmailMessage is only partially correct, because the EmailMultiAlternatives introduces some properties and methods of its own, such as alternatives. If you access any of them, the type checker will complain. I think, thus, the correct type is:

outbox: list[EmailMessage | EmailMultiAlternatives]

[alexei]

On another note, I'm not sure EmailMultiAlternatives.alternatives is correctly typed as list[tuple[bytes | EmailMessage | Message | SafeMIMEText | str, str]], when Django seems to create a list of EmailAlternatives:

https://github.com/django/django/blob/85586052e8d1d9f160b9f1b351e61a787a8e4bed/django/core/mail/message.py#L479-L481