Set fixed Python version for docs

Author: Bernardo-MG

.github/workflows/deploy_docs.yml

@@ -18,10 +18,10 @@ jobs:
       id: extract_branch
     - name: Check-out
       uses: actions/checkout@v2
-    - name: Set up Python ${{ matrix.python-version }}
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: ${{ matrix.python-version }}
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip