I just use CherryPy. It can run under WSGI, be a host for WSGI applications (and is usually used to host Flask applications) and runs it's own web server as well. CherryPy is pretty decent and performant, both Hulu and Netflix use it internally or have in the past. I do know one of the main devs is trying to make it support more of the asyncio stuff, though that might be a while.

To answer your question as to "why WSGI?" the point is that if you write code that targets WSGI, no matter what you can put your Python code behind any WSGI compliant server. You wont be screwed because you picked a dying framework.