Move Sphinx pin to tox.ini

Turns out the Sphinx installed "outside" of the tox virtualenv is not
the one that actually builds the docs...

.github/workflows/ci.yaml

@@ -86,7 +86,7 @@ jobs:
       - name: Install base dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install tox 'sphinx<4.4.0'
+          python -m pip install tox sphinx
 
       - name: Add problem matcher
         run: echo "::add-matcher::.github/sphinx-problem-matcher.json"

tox.ini

@@ -24,7 +24,7 @@ commands =
 
 [testenv:docs]
 basepython = python3.9
-deps = sphinx
+deps = sphinx<4.4.0
 commands = sphinx-build -W -q -b html docs {envtmpdir}/html {posargs}
 
 # checks all links in the docs