Fix Python version issue for the API reference build

Author: gustavocidornelas

.github/workflows/docs.yml

@@ -13,6 +13,9 @@ jobs:
       - uses: actions/checkout@v1
       # https://github.com/marketplace/actions/setup-python
       # ^-- This gives info on matrix testing.
+      - uses: actions/setup-python@v4
+        with:
+          python-version: "3.9"
       - uses: ammaraskar/sphinx-action@master
         with:
           pre-build-command: "pip install --upgrade pip; pip install -e ."

docs/requirements.txt

@@ -1,6 +1,6 @@
 numpydoc
 pydata-sphinx-theme==0.14.1
-sphinx>=7.2.6
+sphinx==7.2.6
 # These packages cause 'make' to break
 protobuf==3.20.2
 pygments>=2.7.0