`environ['werkzeug.server.shutdown']` function is deprecated

[anders-kiaer]

This line

dash/dash/testing/application_runners.py

Line 119 in 66c7847

stopper = flask.request.environ.get("werkzeug.server.shutdown")

gives

[...]/python3.7/site-packages/dash/testing/application_runners.py:122: UserWarning:
  The 'environ['werkzeug.server.shutdown']' function is deprecated and will be removed in Werkzeug 2.1.

See pallets/werkzeug#1873 and https://werkzeug.palletsprojects.com/en/2.0.x/serving/#shutting-down-the-server.

The function is said to be removed in Werkzeug 2.1 (Werkzeug is currently at 2.0.1).

Related issue over at plotly/jupyter-dash#63.