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* Fix outdated docs content and links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_settings/urls.py

@@ -1,5 +1,5 @@
-from django.conf.urls import include, url
+from django.urls import include, re_path
 
 urlpatterns = [
-    url(r'^anymail/', include('anymail.urls')),
+    re_path(r'^anymail/', include('anymail.urls')),
 ]