Make all public is_ functions into methods instead of properties.

examples/background_task.py

@@ -18,7 +18,7 @@ class MyClient(discord.Client):
         await self.wait_until_ready()
         counter = 0
         channel = self.get_channel(1234567) # channel ID goes here
-        while not self.is_closed:
+        while not self.is_closed():
             counter += 1
             await channel.send(counter)
             await asyncio.sleep(60) # task runs every 60 seconds