Create WSGI middleware

[Archmonger]

Current Situation

We have a PR that added ASGI middleware. This PR did not include WSGI middleware to limit scope. However, it is technologically possible for us to run on WSGI.

Proposed Actions

Add WSGI support to ReactPy, which will involve utilizing the simple_websocket package for websocket communication.

We used to have this support for WSGI in our legacy flask backend. WSGI middleware design might take some inspiration from this legacy design.

We will need a cross-platform wsgi webserver for our test suite, but they are unfortunately kind of rare. We might be able to use fastwsgi if it supports simple_websocket.