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* Fix outdated docs content and links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_webhooks.py

@@ -1,10 +1,10 @@
-import json
-from datetime import datetime
-from six.moves.urllib.parse import urljoin
-
 import hashlib
 import hmac
+import json
 from base64 import b64encode
+from datetime import datetime
+from urllib.parse import urljoin
+
 from django.core.exceptions import ImproperlyConfigured
 from django.test import override_settings, tag
 from django.utils.timezone import utc
@@ -12,8 +12,7 @@ from mock import ANY
 
 from anymail.signals import AnymailTrackingEvent
 from anymail.webhooks.mandrill import MandrillCombinedWebhookView, MandrillTrackingWebhookView
-
-from .webhook_cases import WebhookTestCase, WebhookBasicAuthTestsMixin
+from .webhook_cases import WebhookBasicAuthTestCase, WebhookTestCase
 
 TEST_WEBHOOK_KEY = 'TEST_WEBHOOK_KEY'
 
@@ -65,14 +64,14 @@ class MandrillWebhookSettingsTestCase(WebhookTestCase):
 
 @tag('mandrill')
 @override_settings(ANYMAIL_MANDRILL_WEBHOOK_KEY=TEST_WEBHOOK_KEY)
-class MandrillWebhookSecurityTestCase(WebhookTestCase, WebhookBasicAuthTestsMixin):
+class MandrillWebhookSecurityTestCase(WebhookBasicAuthTestCase):
     should_warn_if_no_auth = False  # because we check webhook signature
 
     def call_webhook(self):
         kwargs = mandrill_args([{'event': 'send'}])
         return self.client.post(**kwargs)
 
-    # Additional tests are in WebhookBasicAuthTestsMixin
+    # Additional tests are in WebhookBasicAuthTestCase
 
     def test_verifies_correct_signature(self):
         kwargs = mandrill_args([{'event': 'send'}])
@@ -112,7 +111,7 @@ class MandrillWebhookSecurityTestCase(WebhookTestCase, WebhookBasicAuthTestsMixi
         response = self.client.post(SERVER_NAME="127.0.0.1", **kwargs)
         self.assertEqual(response.status_code, 200)
 
-    # override WebhookBasicAuthTestsMixin version of this test
+    # override WebhookBasicAuthTestCase version of this test
     @override_settings(ANYMAIL={'WEBHOOK_SECRET': ['cred1:pass1', 'cred2:pass2']})
     def test_supports_credential_rotation(self):
         """You can supply a list of basic auth credentials, and any is allowed"""