Replace flask development server backend with production one

We should stop using flask server and replace it with a production one. Since we are using SocketIO, one of the highest recommended packages is 'eventlet'.

Add 'eventlet' to 'install_requires' in setup.py as a required dependency.

Then update some of the code for socketio in graphex/server.py:

socketio = SocketIO(
    app,
    async_mode="eventlet",            # make the backend explicit
    path="/api/socket.io",
    cors_allowed_origins="*",
    logger=False,
    engineio_logger=False
)

and

self.socketio.run(
    app=self.app,
    host="0.0.0.0",
    port=port,
    ssl_context=self.ssl_context,
    debug=False
)

Then do some testing to determine nothing is broken with executing existing graphs.

Start with setup.py to inspect install_requires and graphex/server.py to trace the SocketIO setup and server run call. Add the required production dependency and apply the specified SocketIO and run configuration. Exercise existing graph execution to confirm that serving and graph execution still work without breakage.