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* Fix outdated docs content and links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_mandrill_backend.py

@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-
 from datetime import date, datetime
 from decimal import Decimal
 from email.mime.base import MIMEBase
@@ -14,7 +12,7 @@ from anymail.exceptions import (AnymailAPIError, AnymailRecipientsRefused,
                                 AnymailSerializationError, AnymailUnsupportedFeature)
 from anymail.message import attach_inline_image
 
-from .mock_requests_backend import RequestsBackendMockAPITestCase, SessionSharingTestCasesMixin
+from .mock_requests_backend import RequestsBackendMockAPITestCase, SessionSharingTestCases
 from .utils import sample_image_content, sample_image_path, SAMPLE_IMAGE_FILENAME, AnymailTestMixin, decode_att
 
 
@@ -30,7 +28,7 @@ class MandrillBackendMockAPITestCase(RequestsBackendMockAPITestCase):
     }]"""
 
     def setUp(self):
-        super(MandrillBackendMockAPITestCase, self).setUp()
+        super().setUp()
         # Simple message useful for many tests
         self.message = mail.EmailMultiAlternatives('Subject', 'Text Body', 'from@example.com', ['to@example.com'])
 
@@ -170,7 +168,7 @@ class MandrillBackendStandardEmailTests(MandrillBackendMockAPITestCase):
         self.assertFalse('images' in data['message'])
 
     def test_unicode_attachment_correctly_decoded(self):
-        self.message.attach(u"Une pièce jointe.html", u'<p>\u2019</p>', mimetype='text/html')
+        self.message.attach("Une pièce jointe.html", '<p>\u2019</p>', mimetype='text/html')
         self.message.send()
         data = self.get_api_call_json()
         attachments = data['message']['attachments']
@@ -610,14 +608,14 @@ class MandrillBackendRecipientsRefusedTests(MandrillBackendMockAPITestCase):
 
 
 @tag('mandrill')
-class MandrillBackendSessionSharingTestCase(SessionSharingTestCasesMixin, MandrillBackendMockAPITestCase):
+class MandrillBackendSessionSharingTestCase(SessionSharingTestCases, MandrillBackendMockAPITestCase):
     """Requests session sharing tests"""
-    pass  # tests are defined in the mixin
+    pass  # tests are defined in SessionSharingTestCases
 
 
 @tag('mandrill')
 @override_settings(EMAIL_BACKEND="anymail.backends.mandrill.EmailBackend")
-class MandrillBackendImproperlyConfiguredTests(SimpleTestCase, AnymailTestMixin):
+class MandrillBackendImproperlyConfiguredTests(AnymailTestMixin, SimpleTestCase):
     """Test backend without required settings"""
 
     def test_missing_api_key(self):