Bump notebook from 5.7.8 to 6.1.5 in /demo/robotlab

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 5.7.8 to 6.1.5.
- [Release notes](https://github.com/jupyter/jupyterhub/releases)
- [Changelog](https://github.com/jupyterhub/jupyterhub/blob/master/CHECKLIST-Release.md)
- [Commits](https://github.com/jupyter/jupyterhub/commits)

Signed-off-by: dependabot[bot] <support@github.com>

demo/robotlab/requirements-python3.txt

@@ -36,7 +36,7 @@ lunr==0.5.5
 mistune==0.8.4
 nbconvert==5.5.0
 nbformat==4.4.0
-notebook==5.7.8
+notebook==6.1.5
 pandocfilters==1.4.2
 parso==0.5.1
 pexpect==4.7.0