build(deps): bump notebook from 6.0.3 to 6.1.5

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 6.0.3 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>

requirements.txt

@@ -7,4 +7,4 @@ jupyter-client==6.1.3
 jupyter-console==6.1.0
 jupyter-core==4.6.3
 nbconvert==5.6.1
-notebook==6.0.3
+notebook==6.1.5