DOC: upper bound on Sphinx

dev-requirements.txt

@@ -1,5 +1,9 @@
 setuptools; python_version >= '3.12'
-sphinx >= 3
+# The ability to build documentation using `python setup.py build_sphinx` 
+# has been removed as of Sphinx v7.
+# Until the setup script `setup.py` has been changed to setup.cfg
+# we cannot use sphinx 7+
+Sphinx >= 3, <7
 sphinx_rtd_theme >= 0.4
 pytest >= 6
 scipy >= 1