Bump notebook from 6.0.0 to 6.1.5

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 6.0.0 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] <[email protected]>

Author: dependabot[bot]

requirements.txt

@@ -33,7 +33,7 @@ mistune==0.8.4
 mizani==0.5.4
 nbconvert==5.5.0
 nbformat==4.4.0
-notebook==6.0.0
+notebook==6.1.5
 numpy==1.16.4
 packaging==19.1
 palettable==3.2.0