Change regex from \d+ to [0-9]+ for performance reasons.

\d+ includes unicode characters while [0-9]+ doesn't.

discord/client.py

@@ -206,7 +206,7 @@ class Client:
         if isinstance(mentions, list):
             return [user.id for user in mentions]
         elif mentions == True:
-            return re.findall(r'<@(\d+)>', content)
+            return re.findall(r'<@([0-9]+)>', content)
         else:
             return []