Fresh 20.0.0 release of gunicorn broke the wsgi application factory feature (see https://github.com/benoitc/gunicorn/issues/2159).
So pin gunicorn to version 19.9.0 until the issue gets fixed upstream.
Related to T2080
Can you please make this 'gunicorn<20' instead so we still get potential updates to the 19.x branch?
Rebase and update commit message