(Merge 3.2) Close #12230: Mac OS X Tiger (10.4) has a kernel bug: sometimes,

the file descriptor of a pipe closed in the parent process is valid in the
child process according to fstat(), but the mode of the file descriptor is
invalid, and read or write raise an error.

test.support.requires_mac_ver() is now a decorator, as suggested by Ezio
Melotti, and its docstring is fixed (linux_version => mac_ver).