github: run workflows with Python 3.9

Author: mgorny

.github/workflows/doc.yml

@@ -11,10 +11,10 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v2
 
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Configure pip cache
       uses: actions/cache@v2

.github/workflows/release.yml

@@ -12,10 +12,10 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v2
 
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Install dependencies
       run: |