[tasks] Allow Loop.cancel in Loop.before_loop

Task cancel raises on the next awaited coro, so I've added this 0-sleep "hack"

I'm internally debating if leaving the comment there, but I'm sure it would confuse the uninformed of this trick.

discord/ext/tasks/__init__.py

@@ -68,6 +68,7 @@ class Loop:
         sleep_until = discord.utils.sleep_until
         self._next_iteration = datetime.datetime.now(datetime.timezone.utc)
         try:
+            await asyncio.sleep(0) # allows canceling in before_loop
             while True:
                 self._last_iteration = self._next_iteration
                 self._next_iteration = self._get_next_sleep_time()