stdlib: Add os.pidfd_open(pid)

stdlib/posix.pyi

@@ -273,7 +273,8 @@ if sys.platform != "win32":
         from os import CLD_KILLED as CLD_KILLED, CLD_STOPPED as CLD_STOPPED, waitstatus_to_exitcode as waitstatus_to_exitcode
 
         if sys.platform == "linux":
-            from os import P_PIDFD as P_PIDFD
+            from os import P_PIDFD as P_PIDFD, pidfd_open as pidfd_open
+
     if sys.version_info >= (3, 8):
         from os import (
             POSIX_SPAWN_CLOSE as POSIX_SPAWN_CLOSE,