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* Fix outdated docs content and links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)

anymail/backends/sendgrid.py

@@ -7,7 +7,7 @@ from requests.structures import CaseInsensitiveDict
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 from ..exceptions import AnymailConfigurationError, AnymailRequestsAPIError, AnymailWarning
 from ..message import AnymailRecipientStatus
-from ..utils import BASIC_NUMERIC_TYPES, Mapping, get_anymail_setting, timestamp, update_deep
+from ..utils import BASIC_NUMERIC_TYPES, Mapping, get_anymail_setting, update_deep
 
 
 class EmailBackend(AnymailRequestsBackend):
@@ -47,7 +47,7 @@ class EmailBackend(AnymailRequestsBackend):
                                       default="https://api.sendgrid.com/v3/")
         if not api_url.endswith("/"):
             api_url += "/"
-        super(EmailBackend, self).__init__(api_url, **kwargs)
+        super().__init__(api_url, **kwargs)
 
     def build_message_payload(self, message, defaults):
         return SendGridPayload(message, defaults, self)
@@ -84,9 +84,7 @@ class SendGridPayload(RequestsPayload):
         http_headers['Authorization'] = 'Bearer %s' % backend.api_key
         http_headers['Content-Type'] = 'application/json'
         http_headers['Accept'] = 'application/json'
-        super(SendGridPayload, self).__init__(message, defaults, backend,
-                                              headers=http_headers,
-                                              *args, **kwargs)
+        super().__init__(message, defaults, backend, headers=http_headers, *args, **kwargs)
 
     def get_api_endpoint(self):
         return "mail/send"
@@ -294,7 +292,7 @@ class SendGridPayload(RequestsPayload):
     def set_send_at(self, send_at):
         # Backend has converted pretty much everything to
         # a datetime by here; SendGrid expects unix timestamp
-        self.data["send_at"] = int(timestamp(send_at))  # strip microseconds
+        self.data["send_at"] = int(send_at.timestamp())  # strip microseconds
 
     def set_tags(self, tags):
         self.data["categories"] = tags