_io.open raises a NotImplementedError instead of returning None

- the method should not be called before the _post_init builtin is imported

Author: cosminbasca

graalpython/lib-graalpython/_io.py

@@ -672,7 +672,7 @@ class TextIOWrapper(_TextIOBase):
 
 def open(*args, **kwargs):
     # this method will be overwritten in _patches
-    pass
+    raise NotImplementedError
 
 
 # set the builtins open method