Update nbsphinx requirement from 0.8.7 to 0.8.8

Updates the requirements on [nbsphinx](https://github.com/spatialaudio/nbsphinx) to permit the latest version.
- [Release notes](https://github.com/spatialaudio/nbsphinx/releases)
- [Changelog](https://github.com/spatialaudio/nbsphinx/blob/master/NEWS.rst)
- [Commits](spatialaudio/nbsphinx@0.8.7...0.8.8)

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

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

pyproject.toml

@@ -64,7 +64,7 @@ sphinx-copybutton = { version = "0.5.0", optional = true }
 enum_tools = { version = "0.9.0", optional = true }
 sphinx-toolbox = { version = "2.17.0", optional = true }
 jupytext = { version = "1.13.7", optional = true }
-nbsphinx = { version = "0.8.7", optional = true }
+nbsphinx = { version = "0.8.8", optional = true }
 
 [tool.poetry.extras]
 test = [