Implement __class_getitem__() for dataclass

Author: ubaidsk

src/runtime/lpython/lpython.py

@@ -44,6 +44,7 @@ def __call__(self, arg):
         return self._convert(arg)
 
 def dataclass(arg):
+    arg.__class_getitem__ = lambda self: None
     return py_dataclass(arg)
 
 def is_dataclass(obj):