Support Python 3.6 with Django 1.11-alpha

Only real problem is in json serialization tests:
Python 3.6 [changed][1] the json serialization
error message to use the object's class name
rather than its repr. E.g.:
  "Decimal('19.99') is not JSON serializable"
becomes:
  "Object of type 'Decimal' is not JSON serializable"

Update tests that looked for specific serialization
error message to just look for the word "Decimal"
instead. (Works with all Python versions.)

[1]: https://bugs.python.org/issue26623

tests/test_sendgrid_v2_backend.py

@@ -163,7 +163,7 @@ class SendGridBackendStandardEmailTests(SendGridBackendMockAPITestCase):
 
     def test_extra_headers_serialization_error(self):
         self.message.extra_headers = {'X-Custom': Decimal(12.5)}
-        with self.assertRaisesMessage(AnymailSerializationError, "Decimal('12.5')"):
+        with self.assertRaisesMessage(AnymailSerializationError, "Decimal"):
             self.message.send()
 
     def test_reply_to(self):
@@ -592,7 +592,7 @@ class SendGridBackendAnymailFeatureTests(SendGridBackendMockAPITestCase):
         err = cm.exception
         self.assertIsInstance(err, TypeError)  # compatibility with json.dumps
         self.assertIn("Don't know how to send this data to SendGrid", str(err))  # our added context
-        self.assertIn("Decimal('19.99') is not JSON serializable", str(err))  # original message
+        self.assertRegex(str(err), r"Decimal.*is not JSON serializable")  # original message
 
 
 class SendGridBackendRecipientsRefusedTests(SendGridBackendMockAPITestCase):