|   | 1 |  initial version  | 
I answer myself. I found that I could directly use pip. The command 
sage -pip install jupyterlab_widgets
did the trick.
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.