Limit Sphinx version to secure platform patches

Avoid major versions which might change the API. We have to check if the platform optionflags still work on newer versions once they are available.

docs/requirements.txt

@@ -19,6 +19,6 @@ psutil
 requests>=2.5.1
 ropgadget>=5.3
 sphinx==1.8.6; python_version<'3'
-sphinx>=7.0.0; python_version>='3'
+sphinx>=8.1.3, <9; python_version>='3'
 sphinx_rtd_theme
 sphinxcontrib-autoprogram<=0.1.5