Use flake8 v6.0 for code quality checks

Author: aspeake

.github/workflows/tests.yml

@@ -22,7 +22,7 @@ jobs:
       - name: Install Python dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install flake8
+          pip install flake8==6.0
       - name: Check Python code quality
         run: flake8 --exclude docs/conf.py --max-line-length=100