[MNT] fix readthedocs build (#1777)

This PR attempts to fix the readthedoc build, which seems to be failing
due to an incompatibility between `sphinx` and `nbsphinx`.

The attemted solution is to bound `sphinx`.

The fix in #1775 is
also required.

pyproject.toml

@@ -106,7 +106,7 @@ dev = [
 
 # docs - dependencies for building the documentation
 docs = [
-  "sphinx>3.2",
+  "sphinx>3.2,<7.2.6",
   "pydata-sphinx-theme",
   "nbsphinx",
   "pandoc",