gevent has deprecated .wsgi, should use .pywsgi instead

https://github.com/gevent/gevent/blob/master/doc/api/gevent.wsgi.rst
(cherry picked from commit c9ca451568)

src/sbin/lorax-composer

@@ -33,7 +33,7 @@ import subprocess
 import tempfile
 from threading import Lock
 from gevent import socket
-from gevent.wsgi import WSGIServer
+from gevent.pywsgi import WSGIServer
 
 from pylorax import vernum
 from pylorax.api.cmdline import lorax_composer_parser