[3.14] gh-141659: Fix bad file descriptor error in subprocess on AIX (GH-141660) (GH-141738)

/proc/self does not exist on AIX.
(cherry picked from commit 92c5de7)

Co-authored-by: Ayappan Perumal <ayappap2@in.ibm.com>

Author: miss-islington

Misc/NEWS.d/next/Core_and_Builtins/2025-11-17-08-16-30.gh-issue-141659.QNi9Aj.rst

@@ -0,0 +1 @@
+Fix bad file descriptor errors from ``_posixsubprocess`` on AIX.

Modules/_posixsubprocess.c

@@ -514,7 +514,13 @@ _close_open_fds_maybe_unsafe(int start_fd, int *fds_to_keep,
         proc_fd_dir = NULL;
     else
 #endif
+#if defined(_AIX)
+        char fd_path[PATH_MAX];
+        snprintf(fd_path, sizeof(fd_path), "/proc/%ld/fd", (long)getpid());
+        proc_fd_dir = opendir(fd_path);
+#else
         proc_fd_dir = opendir(FD_DIR);
+#endif
     if (!proc_fd_dir) {
         /* No way to get a list of open fds. */
         _close_range_except(start_fd, -1, fds_to_keep, fds_to_keep_len,