[tasks] Move the Loop's sleep to be before exit conditions

This change makes it more so that `Loop.stop()` gracefully makes the
current iteration the final one, by waiting AND THEN returning.
The current implementation is closer to `cancel`, while also not.

I encountered this because I was trying to run a
`@tasks.loop(count=1)`, and inside it I print some text and change the
interval, and in an `after_loop`, I restart the loop.

Without this change, it immediately floods my console, due to
not waiting before executing `after_loop`.

discord/ext/tasks/__init__.py

@@ -109,13 +109,13 @@ class Loop:
                         raise
                     await asyncio.sleep(backoff.delay())
                 else:
+                    await sleep_until(self._next_iteration)
+                    
                     if self._stop_next_iteration:
                         return
                     self._current_loop += 1
                     if self._current_loop == self.count:
                         break
-
-                    await sleep_until(self._next_iteration)
         except asyncio.CancelledError:
             self._is_being_cancelled = True
             raise