Bump nbconvert from 6.3.0 to 6.5.1 in /source

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
...

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Author: dependabot[bot]

source/rtd_requirements.txt

@@ -13,4 +13,4 @@ pandas
 pandas-bokeh
 imageio
 
-nbconvert==6.4.4
+nbconvert==6.5.1