The commit which added the checks for os.dup a15afb5e48
suggests it was done for Jython. But pytest doesn't support Jython
anymore (Jython is Python 2 only).
Furthermore, it looks like the faulthandler plugin (bundled in pytest
and enabled by default) uses os.dup() unprotected and there have not
been any complaints.
So seems better to just remove these checks, and only add if someone
with a legitimate use case complains.
Ran Benita