Pin Sphinx <4.4.0 in CI

A recent change in Sphinx introduced a new warning about missed extlink
opportunities:
sphinx-doc/sphinx#10112

These warnings are causing spurious CI failures. Some of these
suggestions are good but many of them are not, and there is not
currently a way to disable the warning (globally or locally). So the
only workable solution currently seems to be to pin an old version of
Sphinx in CI for now. Hopefully there will be an option to disable this
in 4.4.1, at which point we can unpin.

.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
+          python -m pip install tox 'sphinx<4.4.0'
 
       - name: Add problem matcher
         run: echo "::add-matcher::.github/sphinx-problem-matcher.json"