CI: pin flake8 version

To avoid new flake8 rules being applied at random times.

.github/workflows/python.yml

@@ -20,7 +20,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install flake8 flake8-import-order
+          python -m pip install flake8==5.0.4 flake8-import-order
 
       - name: Lint with flake8
         run: |