Fix GitHub workflow problem

Python 3.9.13 has spontaneously stopped working on GitHub for no
reason that I understand. It worked yesterday. Change to 3.9.19.

.github/workflows/flake8.yml

@@ -11,7 +11,7 @@ jobs:
     - name: Set up Python 3.9
       uses: actions/setup-python@v4
       with:
-        python-version: '3.9.13'
+        python-version: '3.9.19'
     - name: Update
       run: sudo apt update
     - name: Install apt dependencies
@@ -42,7 +42,7 @@ jobs:
     - name: Set up Python 3.9
       uses: actions/setup-python@v4
       with:
-        python-version: '3.9.13'
+        python-version: '3.9.19'
     - name: Display python version
       run: python -c "import sys; print(sys.version)"
     - name: Update