Bump nbsphinx from 0.9.3 to 0.9.4

Bumps [nbsphinx](https://github.com/spatialaudio/nbsphinx) from 0.9.3 to 0.9.4.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.9.3...0.9.4)

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

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