bpo-38692: Skip test_posix.test_pidfd_open() on EPERM (GH-17290)

Skip the test_posix.test_pidfd_open() test if os.pidfd_open() fails
with a PermissionError. This situation can happen in a Linux sandbox
using a syscall whitelist which doesn't allow the pidfd_open()
syscall yet (like systemd-nspawn).