Cap sphinxcontrib-*help and alabaster versions in tests.

They don't correctly specify their minimum sphinx requirement.

tox.ini

@@ -69,6 +69,13 @@ deps =
     sphinx3.3: sphinx~=3.3.0
     sphinx3.4: sphinx~=3.4.0
     sphinx3.5: sphinx~=3.5.0
+    sphinxcontrib-applehelp<=1.0.4
+    sphinxcontrib-devhelp<=1.0.2
+    sphinxcontrib-htmlhelp<=2.0.1
+    sphinxcontrib-jsmath<=1.0.1
+    sphinxcontrib-qthelp<=1.0.3
+    sphinxcontrib-serializinghtml<=1.1.5
+    alabaster<=0.7.13
 commands =
     python --version
     python -m pytest --cov=sphinx_highlights -r aR tests/ {posargs}