Temporarily pin down Sphinx to work around RTD bug (#434)

* readthedocs/readthedocs.org#10279
* https://pullanswer.com/questions/project-fails-to-build-with-sphinx-7-0-0-jinja2-exceptions-undefinederror-style-is-undefined

Author: DimitriPapadopoulos

pyproject.toml

@@ -47,7 +47,7 @@ Homepage = "https://github.com/zarr-developers/numcodecs"
 
 [project.optional-dependencies]
 docs = [
-    "sphinx",
+    "sphinx<7.0.0",
     "sphinx-issues",
     "numpydoc",
     "mock",