Bump nbconvert from 6.4.2 to 6.4.4

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 6.4.2 to 6.4.4.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](https://github.com/jupyter/nbconvert/compare/6.4.2...6.4.4)

---
updated-dependencies:
- dependency-name: nbconvert
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

requirements-dev.txt

@@ -17,7 +17,7 @@ isort==5.10.1
 time-machine==2.6.0
 
 # Convert jupyter notebooks to markdown documents
-nbconvert==6.4.2
+nbconvert==6.4.4
 
 # mypy types
 types-cachetools==4.2.10