Pin Sphinx to avoid bullet rendering bug

docs/requirements.txt

@@ -1,3 +1,7 @@
 cython>=0.20
 numpy>=1.9
+
+# Note: beware of https://github.com/readthedocs/sphinx_rtd_theme/issues/1115
+# and broken behavior with docutils 0.17
 sphinx_rtd_theme>=1
+sphinx=5.3.0