Fix installing dependencies from requirements.txt instead of backend/requirements.txt

.github/workflows/python-app.yml

@@ -24,7 +24,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install flake8 pytest
-        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        if [ -f backend/requirements.txt ]; then pip install -r backend/requirements.txt; fi
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names