Add requests as dependency because fastapi's TestClient needs it

.github/workflows/python-app.yml

@@ -23,7 +23,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install flake8 pytest
+        pip install flake8 pytest requests
         if [ -f backend/requirements.txt ]; then pip install -r backend/requirements.txt; fi
     - name: Lint with flake8
       run: |