Feature: Switch to a modern server like granian

[reneleonhardt]

gunicorn doesn't support modern HTTP protocol versions like 2 and 3.
https://docs.gunicorn.org/en/stable/news.html#id2

HTTP versions < 1.0 or >= 2.0 are no longer accepted by default (rare, only HTTP/1.1 is supported)

granian seems to be the most popular alternative which already supports HTTP/2 (and HTTP/3 in the future), modern interfaces like ASGI/3 and RSGI and features like direct static file serving:
https://github.com/emmett-framework/granian