remove flask-script

flask-script has been deprecated by the internal flask.cli module, but
making this carries a few changes with it

* you should add FLASK_APP=application.py and FLASK_DEBUG=1 to your
  environment.sh.
* instead of using `python app.py runserver`, now you must run
  `flask run -p 6012`. The -p command is important - the port must be
  set before the config is loaded, so that it can live reload nicely.
  (https://github.com/pallets/flask/issues/2113#issuecomment-268014481)
* find available commands by just running `flask`.
* run them using flask. eg `flask list_routes`
* define new tasks by giving them the decorator
  `@app.cli.command('task-name')`. Task name isn't needed if it's just
  the same as the function name. Alternatively, if app isn't available
  in the current scope, you can invoke the decorator directly, as seen
  in app/commands.py

tests/conftest.py

@@ -5,7 +5,7 @@ from unittest.mock import Mock
 
 import pytest
 from notifications_python_client.errors import HTTPError
-from flask import url_for
+from flask import url_for, Flask
 from bs4 import BeautifulSoup
 
 from app import create_app
@@ -31,17 +31,16 @@ from . import (
 
 @pytest.fixture(scope='session')
 def app_(request):
-    app = create_app()
+    app = Flask('app')
+    create_app(app)
 
     ctx = app.app_context()
     ctx.push()
 
-    def teardown():
-        ctx.pop()
-
-    request.addfinalizer(teardown)
     app.test_client_class = TestClient
-    return app
+    yield app
+
+    ctx.pop()
 
 
 @pytest.fixture(scope='function')