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

@@ -48,7 +48,7 @@ msgpack==0.6.1
 nbconvert==5.5.0
 nbformat==4.4.0
 networkx==2.3
-notebook==6.0.0
+notebook==6.1.5
 numba==0.45.1
 numpy==1.16.4
 numpy-groupies==0.9.7