Bump notebook from 5.7.4 to 5.7.8 in /alpha

Bumps [notebook](https://github.com/jupyter/jupyterhub) from 5.7.4 to 5.7.8.
- [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]

alpha/requirements.txt

@@ -69,7 +69,7 @@ nbdiff==1.0.3
 nbformat==4.4.0
 networkx==2.2
 nose==1.3.7
-notebook==5.7.4
+notebook==5.7.8
 numpy==1.16.1
 oauth2client==4.1.3
 openpyxl==2.4.4