Tests: update Postmark integration

Postmark has changed their error message
for invalid server token. (Also, we don't
care about the specific error code, just
that the error is caught and reported.)

tests/test_postmark_integration.py

@@ -132,9 +132,8 @@ def test_template(self):
 
     @override_settings(ANYMAIL_POSTMARK_SERVER_TOKEN="Hey, that's not a server token!")
     def test_invalid_server_token(self):
-        with self.assertRaises(AnymailAPIError) as cm:
+        # Message will include something like
+        # "Request does not contain a valid Server token"
+        # or "Please verify that you are using a valid token"
+        with self.assertRaisesRegex(AnymailAPIError, r"valid.*token"):
             self.message.send()
-        err = cm.exception
-        self.assertEqual(err.status_code, 401)
-        # Make sure the exception message includes Postmark's response:
-        self.assertIn("Please verify that you are using a valid token", str(err))