* Add general instructions for sending AMP Email
with Django
* Document ability of Amazon SES and SendGrid backends
to send AMPHTML (via arbitrary alternative parts)
* Add AMP Email row to ESP support table
Minimum supported versions are now Django 2.0, Python 3.5.
This touches a lot of code, to:
* Remove obsolete portability code and workarounds
(six, backports of email parsers, test utils, etc.)
* Use Python 3 syntax (class defs, raise ... from, etc.)
* Correct inheritance for mixin classes
* Fix outdated docs content and links
* Suppress Python 3 "unclosed SSLSocket" ResourceWarnings
that are beyond our control (in integration tests due to boto3,
python-sparkpost)
Handle MIME attachments with Content-ID as inline by default.
Treat MIME attachments that have a *Content-ID* but no explicit *Content-Disposition*
header as inline, matching the behavior of many email clients.
Django's SMTP EmailBackend allows spoofing the To header by setting
`message.extra_headers["To"]`` different from `message.to`.
No current Anymail ESP supports this. Treat extra_headers["To"] as
an unsupported ESP feature, to flag attempts to use it.
Also document Anymail's special header handling that replicates
Django's SMTP EmailBackend behavior.
* Add filename param to attach_inline_image
* Add attach_inline_image_file function
(parallels EmailMessage.attach and attach_file)
* Use `Content-Disposition: inline` to decide
whether an attachment should be handled inline
(whether or not it's an image, and whether or not
it has a Content-ID)
* Stop conflating filename and Content-ID, for
ESPs that allow both. (Solves problem where
Google Inbox was displaying inline images
as attachments when sent through SendGrid.)
