Upgrade django.core.mail to use Python's modern email API

[Mike Edmunds]

django.core.mail.EmailMessage currently uses ​Python's "legacy" email API. Updating that code to Python's newer ​"modern" email API would fix bugs and substantially simplify it.

​django-developers discussion

The update should be mostly transparent for users of Django's documented mail APIs, but will rework the internals of django.core.mail.message in ways that may require changes to some third-party email backends and other code that borrows undocumented (but popular) functions from that module. In addition, parts of Django's mail test suite will need reworking to avoid dependence on legacy implementation specifics.

Some relevant posts from django-developers:

• ​Background and rationale for switching to Python's modern email API
• ​Proposed implementation details (some of these were modified in later discussion—in particular, we ​don't expect to combine EmailMultiAlternatives into EmailMessage as part of this ticket, and we ​won't deprecate MIMEBase attachments just yet)
• ​Deprecation details for the documented BadHeaderError, SafeMIMEText and SafeMIMEMultipart, and the undocumented forbid_multi_line_headers() and sanitize_address()

[YashRaj1506]

I will try to solve this.

[Mike Edmunds]

@YashRaj1506 I have a bunch of work underway on this and will open a PR soon. Don't want you to needlessly duplicate effort. Code review on the PR would be very welcome once it's ready.

Part of the delay is I've run into a handful of open issues in Python's modern email API that may require further discussion:

• Modern set_content() forces a trailing newline on text content (​https://github.com/python/cpython/issues/121515). I think this is probably fine, but it touches a lot of tests.
• Security issues in modern header serialization (​https://github.com/python/cpython/issues/80222, ​https://github.com/python/cpython/issues/121284). I think these would block this change.

Also trying to decide how best to handle Django's EmailMessage.encoding. It's not documented, but a bunch of tests cover setting it and checking the results. (The only relevant docs are ​this note about the DEFAULT_CHARSET setting also applying to django.core.mail.)

[Mike Edmunds]

I've opened two sequential PRs, as suggested in the django-developers discussion. Both are ready for review:

• ​https://github.com/django/django/pull/18502 improves the test suite for django.core.mail while staying on the existing (legacy) API.
• ​https://github.com/medmunds/django/pull/2 implements the changeover to the modern email API.

(The second PR is in my own Django fork so that it can be based on the first PR without overlap. I'll reopen it upstream once the first PR is closed. But reviews are welcome now if you don't mind working in my fork; just note any discussion history there won't transfer to the eventual upstream PR.)

Some additional notes are in the PR comments.

[Sarah Boyce <42296566+sarahboyce@…>]

In 889be2f:

Refs #35581 -- Clarified some test names and comments in mail tests.

[Sarah Boyce <42296566+sarahboyce@…>]

In 4d76adfa:

Refs #35581 -- Verified attachments in the generated message in mail tests.

This also removed send() calls, as this doesn't check the serialized content, and
the backend tests cover sending.

[Sarah Boyce <42296566+sarahboyce@…>]

In 00861c4c:

Refs #35581 -- Identified mail tests that check for Python 2 behavior.

This also removed a duplicate CTE case (that used to be distinct in Python 2).

[Sarah Boyce <42296566+sarahboyce@…>]

In cf4d902:

Refs #35581 -- Reduced boilerplate in mail tests.