Bump notebook from 6.0.2 to 6.4.1

Bumps [notebook](http://jupyter.org) from 6.0.2 to 6.4.1.

---
updated-dependencies:
- dependency-name: notebook
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -53,7 +53,7 @@ mypy==0.761
 mypy-extensions==0.4.3
 nbconvert==5.6.1
 nbformat==4.4.0
-notebook==6.0.2
+notebook==6.4.1
 oauth2-provider==0.0
 oauthlib==2.0.7
 pandocfilters==1.4.2