How to set `Procfile` for a Mezzanine app deployed on Heroku

Question

I already have a Mezzanine app deployed on Heroku. The system ran OK before, but today the web page showed an error message Internal Server Error. Checked with heroku logs and found that the command web python manage.py run_gunicorn -b 0.0.0.0:$PORT -w 1 is deprecated and Heroku suggests using <projName>.wsgi:application.

But Mezzanine currently has different project layout from Django (Mezzanine's settings.py is in the project root directory), so I tried the following in Procfile:

web: gunicorn wsgi --log-file -

However, such a setting results in error message:

ImportError: No module named 'app'
...
ImportError: Could not import settings 'app.settings' (Is it on sys.path? Is there an import error in the settings file?) No module named 'app'.

Then I tried:

web: gunicorn <projName>.wsgi --log-file -

The error message (of course ;-):

ImportError: No module named '<projName>'

So, how do I set Procfile so that gunicorn is able to find the project's settings.py and wsgi.py?