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* Fix outdated docs content and links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)
Make `AnymailInboundMessage.text`, `.html` and `.get_content_text()`
usually do the right thing for non-UTF-8 messages/attachments. Fixes
an incorrect UnicodeDecodeError when receiving an (e.g.,) ISO-8859-1
encoded message, and improves handling for inbound messages that were
not properly encoded by the sender.
* Decode using the message's (or attachments's) declared charset
by default (rather than always defaulting to 'utf-8'; you can
still override with `get_content_text(charset=...)`
* Add `errors` param to `get_content_text()`, defaulting to 'replace'.
Mis-encoded messages will now use the Unicode replacement character
rather than raising errors. (Use `get_content_text(errors='strict')`
for the previous behavior.)
