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* Fix outdated docs content and links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)

runtests.py

@@ -4,7 +4,6 @@
 #   or
 # runtests.py [tests.test_x tests.test_y.SomeTestCase ...]
 
-from __future__ import print_function
 import sys
 from distutils.util import strtobool
 
@@ -33,7 +32,6 @@ def setup_and_run_tests(test_labels=None):
 
     warnings.simplefilter('default')  # show DeprecationWarning and other default-ignored warnings
 
-    # noinspection PyStringFormat
     os.environ['DJANGO_SETTINGS_MODULE'] = \
         'tests.test_settings.settings_%d_%d' % django.VERSION[:2]
     django.setup()