Bump nbsphinx from 0.9.1 to 0.9.4

Bumps [nbsphinx](https://github.com/spatialaudio/nbsphinx) from 0.9.1 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@0.9.1...0.9.4)

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

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

doc/requirements.txt

@@ -5,7 +5,7 @@ sphinx>=6
 MarkupSafe==2.1.5
 readthedocs-sphinx-ext==2.2.0
 nbformat==5.10.4
-nbsphinx==0.9.1
+nbsphinx==0.9.4
 typing_extensions==4.5.0
 sphinx_rtd_theme==2.0.0
 nbclient==0.10.0