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* Fix outdated docs content and links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/sending/templates.rst

@@ -211,7 +211,7 @@ for use as merge data:
     # Do something this instead:
     message.merge_global_data = {
         'PRODUCT': product.name,  # assuming name is a CharField
-        'TOTAL_COST': "%.2f" % total_cost,
+        'TOTAL_COST': "{cost:0.2f}".format(cost=total_cost),
         'SHIP_DATE': ship_date.strftime('%B %d, %Y')  # US-style "March 15, 2015"
     }