Bump nbconvert from 6.3.0 to 6.5.1 in /tests

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 6.3.0 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@6.3.0...6.5.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

tests/requirements.txt

@@ -1,7 +1,7 @@
 flake8==3.9.2
 isort==5.7.0
 mock==4.0.3
-nbconvert==6.3.0
+nbconvert==6.5.1
 pre-commit==2.13.0
 pytest==6.2.5
 pytest-asyncio==0.18.3