Update notify to expect a list of string targets instead of a single … (#3548)

* Update notify to expect a list of string targets instead of a single string

* Actually do the thing I set out to do

* Fix notify.group test to expect an array of targets

* REST platform will only use the first target in the list

* Update notify platforms to expect a list of targets

* Update notify services.yaml

homeassistant/components/notify/aws_sns.py

@@ -70,9 +70,6 @@ class AWSSNS(BaseNotificationService):
             _LOGGER.info("At least 1 target is required")
             return
 
-        if not isinstance(targets, list):
-            targets = [targets]
-
         message_attributes = {k: {"StringValue": json.dumps(v),
                                   "DataType": "String"}
                               for k, v in kwargs.items() if v}