Migrated from waitress to gunicorn.

`waitress`, unlike `gunicorn`, doesn't provide an easy way to plug into
a WSGI socket that can be used for the websocket interface.

docs/source/conf.py

@@ -201,6 +201,7 @@ autodoc_default_options = {
 }
 
 autodoc_mock_imports = [
+    'gunicorn',
     'googlesamples.assistant.grpc.audio_helpers',
     'google.assistant.embedded',
     'google.assistant.library',