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