Replace Gunicorn with Granian

[IngmarStein]

https://github.com/emmett-framework/granian is a modern HTTP server for Python applications. Compared to the venerable Gunicorn, it offers much better performance, and an option to switch from the legacy WSGI protocol to ASGI. Regarding the latter, there's an easy upgrade path for Flask apps: https://flask.palletsprojects.com/en/stable/deploying/asgi/.

[IngmarStein]

I'm waiting for emmett-framework/granian#97 to avoid breaking people who use Gunicorn to serve over unix sockets.

[tavdog]

No worries. I just wanted to assign the issues that I wasn't going to do myself so I could sort through them easily.