Bump notebook from 6.1.5 to 6.4.1 in /docs_rtd

Bumps [notebook](http://jupyter.org) from 6.1.5 to 6.4.1.

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

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

docs_rtd/requirements.txt

@@ -40,7 +40,7 @@ MarkupSafe==1.1.1
 mistune==0.8.4
 nbconvert==5.6.1
 nbformat==5.0.7
-notebook==6.1.5
+notebook==6.4.1
 numpy==1.19.1
 packaging==20.4
 pandas==1.0.5