model: notification: don't round-trip via list if you want a set 'create' in NotificationModel starts from an empty list, appends entries to it, then converts it to a set. You could equally start with a set and add to it, avoiding the final conversion.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.