Install dependencies in GH Action with pipenv correctly

Accidentally forgot that we're not using requirements.txt anymore. This should be the correct way of using pipenv. (?)

.github/workflows/python-app.yml

@@ -27,8 +27,7 @@ jobs:
       
     - name: Install dependencies
       run: |
-        # python -m pip install --upgrade pip
-        pipenv install -r requirements.txt
+        pipenv install --dev
         
     - name: Run tests
       run: pipenv run pytest