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* Fix outdated docs content and links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)

anymail/webhooks/mandrill.py

@@ -7,14 +7,14 @@ from base64 import b64encode
 from django.utils.crypto import constant_time_compare
 from django.utils.timezone import utc
 
-from .base import AnymailBaseWebhookView
+from .base import AnymailBaseWebhookView, AnymailCoreWebhookView
 from ..exceptions import AnymailWebhookValidationFailure
 from ..inbound import AnymailInboundMessage
 from ..signals import inbound, tracking, AnymailInboundEvent, AnymailTrackingEvent, EventType
 from ..utils import get_anymail_setting, getfirst, get_request_uri
 
 
-class MandrillSignatureMixin(object):
+class MandrillSignatureMixin(AnymailCoreWebhookView):
     """Validates Mandrill webhook signature"""
 
     # These can be set from kwargs in View.as_view, or pulled from settings in init:
@@ -22,29 +22,26 @@ class MandrillSignatureMixin(object):
     webhook_url = None  # optional; defaults to actual url used
 
     def __init__(self, **kwargs):
-        # noinspection PyUnresolvedReferences
         esp_name = self.esp_name
         # webhook_key is required for POST, but not for HEAD when Mandrill validates webhook url.
         # Defer "missing setting" error until we actually try to use it in the POST...
         webhook_key = get_anymail_setting('webhook_key', esp_name=esp_name, default=None,
                                           kwargs=kwargs, allow_bare=True)
         if webhook_key is not None:
-            self.webhook_key = webhook_key.encode('ascii')  # hmac.new requires bytes key in python 3
+            self.webhook_key = webhook_key.encode('ascii')  # hmac.new requires bytes key
         self.webhook_url = get_anymail_setting('webhook_url', esp_name=esp_name, default=None,
                                                kwargs=kwargs, allow_bare=True)
-        # noinspection PyArgumentList
-        super(MandrillSignatureMixin, self).__init__(**kwargs)
+        super().__init__(**kwargs)
 
     def validate_request(self, request):
         if self.webhook_key is None:
             # issue deferred "missing setting" error (re-call get-setting without a default)
-            # noinspection PyUnresolvedReferences
             get_anymail_setting('webhook_key', esp_name=self.esp_name, allow_bare=True)
 
         try:
             signature = request.META["HTTP_X_MANDRILL_SIGNATURE"]
         except KeyError:
-            raise AnymailWebhookValidationFailure("X-Mandrill-Signature header missing from webhook POST")
+            raise AnymailWebhookValidationFailure("X-Mandrill-Signature header missing from webhook POST") from None
 
         # Mandrill signs the exact URL (including basic auth, if used) plus the sorted POST params:
         url = self.webhook_url or get_request_uri(request)