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* Fix outdated docs content and links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)

tests/test_amazon_ses_inbound.py

@@ -1,5 +1,3 @@
-from __future__ import unicode_literals
-
 import json
 from base64 import b64encode
 from datetime import datetime
@@ -22,7 +20,7 @@ from .webhook_cases import WebhookTestCase
 class AmazonSESInboundTests(WebhookTestCase, AmazonSESWebhookTestsMixin):
 
     def setUp(self):
-        super(AmazonSESInboundTests, self).setUp()
+        super().setUp()
         # Mock boto3.session.Session().client('s3').download_fileobj
         # (We could also use botocore.stub.Stubber, but mock works well with our test structure)
         self.patch_boto3_session = patch('anymail.webhooks.amazon_ses.boto3.session.Session', autospec=True)
@@ -263,9 +261,8 @@ class AmazonSESInboundTests(WebhookTestCase, AmazonSESWebhookTestsMixin):
         self.assertEqual([str(to) for to in message.to],
                          ['Recipient <inbound@example.com>', 'someone-else@example.org'])
         self.assertEqual(message.subject, 'Test inbound message')
-        # rstrip below because the Python 3 EmailBytesParser converts \r\n to \n, but the Python 2 version doesn't
-        self.assertEqual(message.text.rstrip(), "It's a body\N{HORIZONTAL ELLIPSIS}")
-        self.assertEqual(message.html.rstrip(), """<div dir="ltr">It's a body\N{HORIZONTAL ELLIPSIS}</div>""")
+        self.assertEqual(message.text, "It's a body\N{HORIZONTAL ELLIPSIS}\n")
+        self.assertEqual(message.html, """<div dir="ltr">It's a body\N{HORIZONTAL ELLIPSIS}</div>\n""")
         self.assertIsNone(message.spam_detected)
 
     def test_inbound_s3_failure_message(self):