bpo-38692: Add os.pidfd_open. (GH-17063)

diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index 17e4ded2..98a39c3 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -1470,6 +1470,15 @@
         open(fn, 'wb').close()
         self.assertRaises(ValueError, os.stat, fn_with_NUL)
 
+    @unittest.skipUnless(hasattr(os, "pidfd_open"), "pidfd_open unavailable")
+    def test_pidfd_open(self):
+        with self.assertRaises(OSError) as cm:
+            os.pidfd_open(-1)
+        if cm.exception.errno == errno.ENOSYS:
+            self.skipTest("system does not support pidfd_open")
+        self.assertEqual(cm.exception.errno, errno.EINVAL)
+        os.close(os.pidfd_open(os.getpid(), 0))
+
 class PosixGroupsTester(unittest.TestCase):
 
     def setUp(self):