Ned Deily added the comment:
> FD_CLOEXEC is first support on OS X 10.5.  Here's a patch to skip the test on earlier systems: tested on 10.4, 10.5, and 10.10.

What do you mean by "first support"? Does it mean that fcntl(fd, F_SETFD, FD_CLOEXEC) is simply a no-op?

Does Python 3.4 with the PEP 446 works on OS X 10.4? Can you test the following example?

Python 3.4.1 (default, Nov  3 2014, 14:38:10)
[GCC 4.9.1 20140930 (Red Hat 4.9.1-11)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> fd=os.open('.', os.O_RDONLY)
>>> os.get_inheritable(fd)
False
>>> os.set_inheritable(fd, True)
>>> os.get_inheritable(fd)
True