fix: use the 3.9 Python version in the 'deploy-docs' workflow

Author: andrii-bodnar

.github/workflows/deploy-main-docs.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v2
         with:
-          python-version: '3.8'
+          python-version: '3.9'
 
       - name: Cache pip
         uses: actions/cache@v2