The proper answer for May 2020: flask-restplus is dead, move to flask-restx.
From noirbizarre/flask-restplus#778 (comment):
flask-restplus work has been discontinued due to maintainers not having pypi keys. See the drop in replacement, flask-restx. It’s an official fork by the maintainer team. We have already fixed the issue there
From noirbizarre/flask-restplus#777 (comment):
No. Flask-restplus is no longer maintained. The former maintainers do not have privileges to push to pypi, and after many months of trying, we forked the project. Check out flask-restx. It’s a drop in replacement and we are roadmapping, designing, and making fixes…for instance, we already patched for Werkzeug
So the real solution is to move to flask-restx rather than pinning to an old version of Werkzeug.