Bump nbconvert from 7.0.0 to 7.2.1

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 7.0.0 to 7.2.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Changelog](https://github.com/jupyter/nbconvert/blob/main/CHANGELOG.md)
- [Commits](https://github.com/jupyter/nbconvert/compare/7.0.0...v7.2.1)

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

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

requirements-dev.txt

@@ -20,7 +20,7 @@ isort==5.10.1
 time-machine==2.8.2
 
 # Convert jupyter notebooks to markdown documents
-nbconvert==7.0.0
+nbconvert==7.2.1
 
 # mypy types
 types-cachetools==5.2.1