Pin Sphinx 5.3.0, refs #1971

Furo is not yet compatible with Sphinx 6.0

setup.py

@@ -66,6 +66,7 @@ setup(
     setup_requires=["pytest-runner"],
     extras_require={
         "docs": [
+            "Sphinx==5.3.0",
             "furo==2022.12.7",
             "sphinx-autobuild",
             "codespell",