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* Fix outdated docs content and links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/tips/django_templates.rst

@@ -7,7 +7,7 @@ ESP's templating languages and merge capabilities are generally not compatible
 with each other, which can make it hard to move email templates between them.
 
 But since you're working in Django, you already have access to the
-extremely-full-featured :mod:`Django templating system <django.template>`.
+extremely-full-featured :doc:`Django templating system <django:topics/templates>`.
 You don't even have to use Django's template syntax: it supports other
 template languages (like Jinja2).
 
@@ -15,7 +15,7 @@ You're probably already using Django's templating system for your HTML pages,
 so it can be an easy decision to use it for your email, too.
 
 To compose email using *Django* templates, you can use Django's
-:func:`~django.template.loaders.django.template.loader.render_to_string`
+:func:`~django.template.loader.render_to_string`
 template shortcut to build the body and html.
 
 Example that builds an email from the templates ``message_subject.txt``,
@@ -24,16 +24,14 @@ Example that builds an email from the templates ``message_subject.txt``,
 .. code-block:: python
 
     from django.core.mail import EmailMultiAlternatives
-    from django.template import Context
     from django.template.loader import render_to_string
 
     merge_data = {
         'ORDERNO': "12345", 'TRACKINGNO': "1Z987"
     }
 
-    plaintext_context = Context(autoescape=False)  # HTML escaping not appropriate in plaintext
-    subject = render_to_string("message_subject.txt", merge_data, plaintext_context)
-    text_body = render_to_string("message_body.txt", merge_data, plaintext_context)
+    subject = render_to_string("message_subject.txt", merge_data).strip()
+    text_body = render_to_string("message_body.txt", merge_data)
     html_body = render_to_string("message_body.html", merge_data)
 
     msg = EmailMultiAlternatives(subject=subject, from_email="store@example.com",
@@ -41,6 +39,9 @@ Example that builds an email from the templates ``message_subject.txt``,
     msg.attach_alternative(html_body, "text/html")
     msg.send()
 
+Tip: use Django's :ttag:`{% autoescape off %}<autoescape>` template tag in your
+plaintext ``.txt`` templates to avoid inappropriate HTML escaping.
+
 
 Helpful add-ons
 ---------------
@@ -48,8 +49,6 @@ Helpful add-ons
 These (third-party) packages can be helpful for building your email
 in Django:
 
-.. TODO: flesh this out
-
 * :pypi:`django-templated-mail`, :pypi:`django-mail-templated`, or :pypi:`django-mail-templated-simple`
   for building messages from sets of Django templates.
 * :pypi:`premailer` for inlining css before sending

docs/tips/securing_webhooks.rst

@@ -73,10 +73,10 @@ Basic usage is covered in the
 :ref:`webhooks configuration <webhooks-configuration>` docs.
 
 If something posts to your webhooks without the required shared
-secret as basic auth in the HTTP_AUTHORIZATION header, Anymail will
+secret as basic auth in the HTTP *Authorization* header, Anymail will
 raise an :exc:`AnymailWebhookValidationFailure` error, which is
 a subclass of Django's :exc:`~django.core.exceptions.SuspiciousOperation`.
-This will result in an HTTP 400 response, without further processing
+This will result in an HTTP 400 "bad request" response, without further processing
 the data or calling your signal receiver function.
 
 In addition to a single "random:random" string, you can give a list