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* Fix outdated docs content and links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/settings_1_11.py

@@ -1,121 +0,0 @@
-"""
-Django settings for Anymail tests.
-
-Generated by 'django-admin startproject' using Django 1.11.
-
-For more information on this file, see
-https://docs.djangoproject.com/en/1.11/topics/settings/
-
-For the full list of settings and their values, see
-https://docs.djangoproject.com/en/1.11/ref/settings/
-"""
-
-import os
-
-# Build paths inside the project like this: os.path.join(BASE_DIR, ...)
-BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
-
-
-# Quick-start development settings - unsuitable for production
-# See https://docs.djangoproject.com/en/1.11/howto/deployment/checklist/
-
-# SECURITY WARNING: keep the secret key used in production secret!
-SECRET_KEY = 'NOT_FOR_PRODUCTION_USE'
-
-# SECURITY WARNING: don't run with debug turned on in production!
-DEBUG = True
-
-ALLOWED_HOSTS = []
-
-
-# Application definition
-
-INSTALLED_APPS = [
-    'django.contrib.admin',
-    'django.contrib.auth',
-    'django.contrib.contenttypes',
-    'django.contrib.sessions',
-    'django.contrib.messages',
-    'django.contrib.staticfiles',
-    'anymail',
-]
-
-MIDDLEWARE = [
-    'django.middleware.security.SecurityMiddleware',
-    'django.contrib.sessions.middleware.SessionMiddleware',
-    'django.middleware.common.CommonMiddleware',
-    'django.middleware.csrf.CsrfViewMiddleware',
-    'django.contrib.auth.middleware.AuthenticationMiddleware',
-    'django.contrib.messages.middleware.MessageMiddleware',
-    'django.middleware.clickjacking.XFrameOptionsMiddleware',
-]
-
-ROOT_URLCONF = 'tests.test_settings.urls'
-
-TEMPLATES = [
-    {
-        'BACKEND': 'django.template.backends.django.DjangoTemplates',
-        'DIRS': [],
-        'APP_DIRS': True,
-        'OPTIONS': {
-            'context_processors': [
-                'django.template.context_processors.debug',
-                'django.template.context_processors.request',
-                'django.contrib.auth.context_processors.auth',
-                'django.contrib.messages.context_processors.messages',
-            ],
-        },
-    },
-]
-
-WSGI_APPLICATION = 'tests.wsgi.application'
-
-
-# Database
-# https://docs.djangoproject.com/en/1.11/ref/settings/#databases
-
-DATABASES = {
-    'default': {
-        'ENGINE': 'django.db.backends.sqlite3',
-        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
-    }
-}
-
-
-# Password validation
-# https://docs.djangoproject.com/en/1.11/ref/settings/#auth-password-validators
-
-AUTH_PASSWORD_VALIDATORS = [
-    {
-        'NAME': 'django.contrib.auth.password_validation.UserAttributeSimilarityValidator',
-    },
-    {
-        'NAME': 'django.contrib.auth.password_validation.MinimumLengthValidator',
-    },
-    {
-        'NAME': 'django.contrib.auth.password_validation.CommonPasswordValidator',
-    },
-    {
-        'NAME': 'django.contrib.auth.password_validation.NumericPasswordValidator',
-    },
-]
-
-
-# Internationalization
-# https://docs.djangoproject.com/en/1.11/topics/i18n/
-
-LANGUAGE_CODE = 'en-us'
-
-TIME_ZONE = 'UTC'
-
-USE_I18N = True
-
-USE_L10N = True
-
-USE_TZ = True
-
-
-# Static files (CSS, JavaScript, Images)
-# https://docs.djangoproject.com/en/1.11/howto/static-files/
-
-STATIC_URL = '/static/'

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')),
 ]