Pin old Sphinx deps to old versions (#6709)

The Sphinx dependencies started to mark their Sphinx version
requirement at runtime, which stopped old Sphinx 4.* from installing
dependencies of the right versions.

pyproject.toml

@@ -65,6 +65,11 @@ test = [
     'Pygments~=2.10.0',
     'Sphinx~=4.2.0',
     'sphinxcontrib-asyncio~=0.3.0',
+    'sphinxcontrib-applehelp<1.0.8',
+    'sphinxcontrib-devhelp<1.0.6',
+    'sphinxcontrib-htmlhelp<2.0.5',
+    'sphinxcontrib-serializinghtml<1.1.10',
+    'sphinxcontrib-qthelp<1.0.7',
     'sphinx_code_tabs~=0.5.3',
 ]