Rewrite using Python 3, Flask and authlib

[abompard]

I think it would be a useful move towards standardization if this app was rewritten to be based on Flask and authlib (instead of Velruse). And Python 3 of course. And it should run in OpenShift (it's currently running in a VM).

Some pointers:

• a cookiecutter template for Flask apps in Fedora Infra : https://github.com/fedora-infra/cookiecutter-flask-webapp
• authlib's Flask integration: https://docs.authlib.org/en/latest/client/flask.html