Bump nbconvert from 5.6.1 to 6.5.1

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 5.6.1 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@5.6.1...6.5.1)

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

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

requirements.txt

@@ -219,6 +219,7 @@ kiwisolver==1.4.3
     # via matplotlib
 lxml==4.9.0
     # via
+    #   nbconvert
     #   trove-newspaper-harvester
     #   trove-newspaper-images
 markupsafe==2.1.1
@@ -245,7 +246,7 @@ nbclient==0.5.13
     # via
     #   nbconvert
     #   voila
-nbconvert==6.5.0
+nbconvert==6.5.1
     # via
     #   jupyter-server
     #   notebook