Bump notebook from 6.4.6 to 6.4.12 in /2021

Bumps [notebook](http://jupyter.org) from 6.4.6 to 6.4.12.

---
updated-dependencies:
- dependency-name: notebook
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]