[commands] Fix flag detection code in get_flags

discord/ext/commands/flags.py

@@ -184,7 +184,7 @@ def get_flags(namespace: Dict[str, Any], globals: Dict[str, Any], locals: Dict[s
 
         annotation = flag.annotation = resolve_annotation(flag.annotation, globals, locals, cache)
 
-        if flag.default is MISSING and issubclass(annotation, FlagConverter) and annotation._can_be_constructible():
+        if flag.default is MISSING and hasattr(annotation, '__commands_is_flag__') and annotation._can_be_constructible():
             flag.default = annotation._construct_default
 
         if flag.aliases is MISSING: