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* Fix outdated docs content and links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/help.rst

@@ -29,9 +29,10 @@ often help you pinpoint the problem...
 **Double-check common issues**
 
   * Did you add any required settings for your ESP to the `ANYMAIL` dict in your
-    settings.py? (E.g., ``"SENDGRID_API_KEY"`` for SendGrid.) See :ref:`supported-esps`.
+    settings.py? (E.g., ``"SENDGRID_API_KEY"`` for SendGrid.) Check the instructions
+    for the ESP you're using under :ref:`supported-esps`.
   * Did you add ``'anymail'`` to the list of :setting:`INSTALLED_APPS` in settings.py?
-  * Are you using a valid from address? Django's default is "webmaster@localhost",
+  * Are you using a valid *from* address? Django's default is "webmaster@localhost",
     which most ESPs reject. Either specify the ``from_email`` explicitly on every message
     you send, or add :setting:`DEFAULT_FROM_EMAIL` to your settings.py.
 
@@ -61,8 +62,8 @@ Support
 
 If you've gone through the troubleshooting above and still aren't sure what's wrong,
 the Anymail community is happy to help. Anymail is supported and maintained by the
-people who use it---like you! (The vast majority of Anymail contributors volunteer
-their time, and are not employees of any ESP.)
+people who use it---like you! (Anymail contributors volunteer their time, and are
+not employees of any ESP.)
 
 Here's how to contact the Anymail community: