Update tests.yml

It helps to install the dependencies when using them

Author: tabbeddev

.github/workflows/tests.yml

@@ -26,7 +26,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install pytest
+        pip install pytest flake8
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Lint with flake8
       run: |