Restrict Sphinx version used for testing

Workaround for incompatibility between the current
sphinx_rtd_theme 1.0.0 and Sphinx 5.2.0.post0

Author: Cito

tox.ini

@@ -44,7 +44,7 @@ commands =
 [testenv:docs]
 basepython = python3.9
 deps =
-    sphinx>=5.1,<6
+    sphinx>=5.1,<5.2
     sphinx_rtd_theme>=1,<2
 commands =
     sphinx-build -b html -nEW docs docs/_build/html