Only defer the lock if we're pre-emptively rate limiting.

If a 429 was already hit somehow then we're no longer pre-emptively
rate limiting and we're going to end up having a race condition in
the lock/unlock cycle.

discord/http.py

@@ -154,7 +154,7 @@ class HTTPClient:
 
                     # check if we have rate limit header information
                     remaining = r.headers.get('X-Ratelimit-Remaining')
-                    if remaining == '0':
+                    if remaining == '0' and r.status != 429:
                         # we've depleted our current bucket
                         if header_bypass_delay is None:
                             now = parsedate_to_datetime(r.headers['Date'])