Fixed #35537 -- Use a namedtuple for email attachments and alternatives

[RealOrangeOne]

This makes it more descriptive to pull out the named fields

Trac ticket number

ticket-35537

Branch description

This change replaces the opaque tuples used for email attachments and alternatives with named tuples. This makes unpacking the specific indexes more descriptive, whilst avoiding breaking any existing uses (since the field order hasn't changed).

email.alternatives[0][0]  # before
email.alternatives[0].content  #after

Unpacking the alternatives doesn't seem to be documented, so there are no docs to update. Similarly, adding attachments / alternatives is done using documented methods, whose public API is unaffected by this change. Manual insertion into the .alternatives and .attachments may now fail, however users should be using the documented API methods.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.

[sarahboyce]

In #18279, I added some docs for EmailMultiAlternatives.
I think it would be nice to add docs for the alternatives attribute here as part of this change 👍

[sarahboyce]

Nice! Have a last couple thoughts

Also in the 5.2 release notes, let's add something like

Email
~~~~~

* Tuple items of ``attachments`` of :class:`~django.core.mail.EmailMessage`
  and :class:`~django.core.mail.EmailMultiAlternatives` are now named tuples,
  as opposed to regular tuples.

* :attr:`EmailMultiAlternatives.alternatives
  <django.core.mail.EmailMultiAlternatives.alternatives>` is now a list of
  named tuples, as opposed to regular tuples.

[sarahboyce]

Thank you 🥳 this looks good to me