GitHub actions: Ignore flake8 no longer supporting Python 3.7 (#2971)

.github/workflows/docs.yml

@@ -23,10 +23,10 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Python 3.8
+      - name: Set up Python 3.9
         uses: actions/setup-python@v1
         with:
-          python-version: 3.8
+          python-version: 3.9
       - uses: actions/cache@v1
         with:
           path: ~/.cache/pip