upgrade python

Author: yosupo06

.github/workflows/document.yml

@@ -16,10 +16,10 @@ jobs:
       - name: Set version name
         id: version
         run: echo "::set-output name=VERSION_NAME::${GITHUB_REF##*/}"
-      - name: Set up Python 3.7
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.9
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip