Drop Python 2 and Django 1.11 support

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* Fix outdated docs content and links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)

docs/esps/mailgun.rst

@@ -425,9 +425,9 @@ The event's :attr:`~anymail.signals.AnymailTrackingEvent.esp_event` field will b
 the parsed `Mailgun webhook payload`_ as a Python `dict` with ``"signature"`` and
 ``"event-data"`` keys.
 
-Anymail uses Mailgun's webhook `token` as its normalized
+Anymail uses Mailgun's webhook ``token`` as its normalized
 :attr:`~anymail.signals.AnymailTrackingEvent.event_id`, rather than Mailgun's
-event-data `id` (which is only guaranteed to be unique during a single day).
+event-data ``id`` (which is only guaranteed to be unique during a single day).
 If you need the event-data id, it can be accessed in your webhook handler as
 ``event.esp_event["event-data"]["id"]``. (This can be helpful for working with
 Mailgun's other event APIs.)

docs/esps/mandrill.rst

@@ -3,7 +3,7 @@
 Mandrill
 ========
 
-Anymail integrates with the `Mandrill <http://mandrill.com/>`__
+Anymail integrates with the `Mandrill <https://mandrill.com/>`__
 transactional email service from MailChimp.
 
 .. note:: **Limited Support for Mandrill**