feat: retry http requests that fail with a 503 status

discord/http.py

@@ -357,8 +357,8 @@ async def request(
 
                             continue
 
-                        # we've received a 500, 502, or 504, unconditional retry
-                        if response.status in {500, 502, 504}:
+                        # we've received a 500, 502, 503, or 504, unconditional retry
+                        if response.status in {500, 502, 503, 504}:
                             await asyncio.sleep(1 + tries * 2)
                             continue