The original patch[1] missed one thing which I just discovered - there's
selectors.DefaultSelector alias for the "the most efficient implementation
available on the current platform"[2].
Before this patch a non-green selector class would be obtained when
DefaultSelector was used.
[1] 0d509ef7d2
[2] https://docs.python.org/3.5/library/selectors.html#selectors.DefaultSelector