Cast Retry-After header into a float.

discord/client.py

@@ -791,7 +791,7 @@ class Client:
         log_fmt = 'In {}, {}'.format(name, tmp)
         log.debug(log_fmt)
         if resp.status == 429:
-            retry = resp.headers['Retry-After'] / 1000.0
+            retry = float(resp.headers['Retry-After']) / 1000.0
             yield from resp.release()
             yield from asyncio.sleep(retry)
             return (yield from self._rate_limit_helper(name, method, data))