fix: set_group put group_type/group_name in groups attribute (#28)

Put group_type and group_name in groups attributes of IdentifyEvent instead of user_properties.

src/amplitude/client.py

@@ -125,9 +125,9 @@ def set_group(self, group_type: str, group_name: Union[str, List[str]], event_op
             event_options (amplitude.event.EventOptions): Provide additional information for event
                 like user_id.
         """
-        identify_obj = Identify()
-        identify_obj.set(group_type, group_name)
-        self.identify(identify_obj, event_options)
+        event = IdentifyEvent(groups={group_type: group_name})
+        event.load_event_options(event_options)
+        self.track(event)
 
     def flush(self):
         """Flush all event waiting to be sent in the buffer

src/test/test_client.py

@@ -182,11 +182,11 @@ def test_amplitude_set_group_success(self):
         def callback_func(event, code, message=None):
             self.assertEqual(200, code)
             self.assertTrue(isinstance(event, IdentifyEvent))
-            self.assertTrue("user_properties" in event)
+            self.assertTrue("groups" in event)
             self.assertEqual("$identify", event["event_type"])
             self.assertEqual("test_user_id", event["user_id"])
             self.assertEqual("test_device_id", event["device_id"])
-            self.assertEqual({"$set": {"type": ["test_group", "test_group_2"]}}, event.user_properties)
+            self.assertEqual({"type": ["test_group", "test_group_2"]}, event.groups)
 
         self.client.configuration.callback = callback_func
         for use_batch in (True, False):