Bump nbconvert from 5.3.1 to 6.5.1 in /xfer-ml/docs/demos (#90)

Bumps [nbconvert](https://github.com/jupyter/nbconvert) from 5.3.1 to 6.5.1.
- [Release notes](https://github.com/jupyter/nbconvert/releases)
- [Commits](jupyter/nbconvert@5.3.1...6.5.1)

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

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xfer-ml/docs/demos/requirements.txt

@@ -1,4 +1,4 @@
 ipython==7.16.3
-nbconvert==5.3.1
+nbconvert==6.5.1
 jupyter>=1.0.0
 emukit>=0.4.5