To get it to move forward: as it is not solely and AIX thing (see bpo-40-140) 

This works: but is it what is wanted?

Tests result: SUCCESS
aixtools@x064:[/home/aixtools/py39-3.9]git diff
diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
index eaada1b504..4a5addc6fe 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -1893,12 +1893,18 @@ class PtyTests(unittest.TestCase):
             self.fail("got %d lines in pipe but expected 2, child output was:\n%s"
                       % (len(lines), child_output))

-        # Wait until the child process completes before closing the PTY to
-        # prevent sending SIGHUP to the child process.
-        support.wait_process(pid, exitcode=0)
+        if sys.platform == "linux" or not os.name == "posix":
+            # Wait until the child process completes before closing the PTY to
+            # prevent sending SIGHUP to the child process.
+            support.wait_process(pid, exitcode=0)

-        # Close the PTY
-        os.close(fd)
+            # Close the PTY
+            os.close(fd)
+        else:
+            # Other posix need to close the pty for the child to exit normally
+            # Close the PTY
+            os.close(fd)
+            support.wait_process(pid, exitcode=0)