Bump notebook from 6.0.2 to 6.1.5

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 6.0.2 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>

Author: dependabot[bot]

requirements.txt

@@ -130,7 +130,7 @@ murmurhash==1.0.2
 nbconvert==5.6.1
 nbformat==4.4.0
 networkx==2.2
-notebook==6.0.2
+notebook==6.1.5
 numba==0.47.0
 numexpr==2.7.0
 numpy==1.16.3