CherryPy support with Python 3

[Alterrien]

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Description

This is a follow-up to #1337.
Using our own fork based on master, we have recently switched to using Python3, and as such have had to move to Gunicorn. Because we rely on SQLAlchemy-based connectors, we have to limit the number of Gunicorn workers to 1 to prevent issues such as described in #3164.

This considerably degraded the performance of Hue to the point that even instances serving ~20 active users and rarely running more than 3 queries in parallel end up with timeouts and gets restarted. We'd like to go back to CherryPy as the code that allows to rely on CherryPy is still there, it just fails because of some Python2 breaking changes.
Thanks

Steps To Reproduce

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Hue version

Open source current master