bump sphinx version to resolve the weird issue with the caption not getting the correct class (Azure#19188)

Author: scbedd

eng/tox/tox.ini

@@ -138,7 +138,7 @@ changedir = {toxinidir}
 passenv = *
 deps =
   {[base]deps}
-  sphinx==3.0.4
+  sphinx==3.5.4
   sphinx_rtd_theme==0.5.0
   recommonmark==0.6.0
   m2r==0.2.1