fix github workflow to install requirements in right order

.github/workflows/tests.yaml

@@ -20,7 +20,7 @@ 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 requirements.txt ]; then xargs -L 1 pip install < requirements.txt; fi
       - name: Lint with flake8
         run: |
           # stop the build if there are Python syntax errors or undefined names