Flask-Restplus is replaced by flask-restx

[dirkpetersen]

Apparently the maintainers had not access to the restplus pypy project so they needed to fork

noirbizarre/flask-restplus#770

and rename the project

https://github.com/python-restx/flask-restx

.... I assume it will be api compatible and we just need to change the import statements

[aicioara]

I don't think there is any need to migrate until we require new features from flask-restplus, which should not be the case. Dead projects are great because they tend to be stable. Also, I would wait for the maintaining team of flask-restx to mature first, it's risky to jump on an arbitrary fork.