bpo-30225: Fix is_valid_fd() on macOS Tiger (#1443) (#1449)

is_valid_fd() now uses fstat() instead of dup() on macOS to return 0
on a pipe when the other side of the pipe is closed. fstat() fails
with EBADF in that case, whereas dup() succeed.
(cherry picked from commit 1c4670ea0cc3d208121af11b9b973e6bb268e570)
1 file changed