zopewsgi: run waitress with asyncore_use_poll by default

on python2, we use have a [python patch][patch] to globally make
asyncore use poll, at that time we were using medusa and it was not
exposing an option to use something else than select.

[patch]: https://lab.nexedi.com/nexedi/slapos/-/blob/180f1553fa7774f05ae8e739dfef43fa2ae56c0a/component/python-2.7/asyncore_poll_insteadof_select.patch

product/ERP5/bin/zopewsgi.py

@@ -157,6 +157,7 @@ def app_wrapper(large_file_threshold, webdav_ports):
 
 def createServer(application, logger, **kw):
     global server
+    kw.setdefault('asyncore_use_poll', True)
     server = create_server(
         TransLogger(application, logger=logger),
         # We handle X-Forwarded-For by ourselves. See ERP5Type/patches/WSGITask.py.