bpo-41818: Close file descriptors in test_openpty (#GH-24119)

When stdin is a TTY, the test added in commit c13d899
is expected to fail. However, when it failed, it did not close
its file descriptors. This is flagged by the refleak tests (but
only when stdin is a TTY, which doesn't seem to be the case on CI).

Lib/test/test_pty.py

@@ -91,7 +91,6 @@ def _set_term_winsz(fd, winsz):
 
 # Marginal testing of pty suite. Cannot do extensive 'do or fail' testing
 # because pty code is not too portable.
-# XXX(nnorwitz):  these tests leak fds when there is an error.
 class PtyTest(unittest.TestCase):
     def setUp(self):
         old_alarm = signal.signal(signal.SIGALRM, self.handle_sig)
@@ -176,6 +175,12 @@ def test_openpty(self):
             # " An optional feature could not be imported " ... ?
             raise unittest.SkipTest("Pseudo-terminals (seemingly) not functional.")
 
+        # closing master_fd can raise a SIGHUP if the process is
+        # the session leader: we installed a SIGHUP signal handler
+        # to ignore this signal.
+        self.addCleanup(os.close, master_fd)
+        self.addCleanup(os.close, slave_fd)
+
         self.assertTrue(os.isatty(slave_fd), "slave_fd is not a tty")
 
         if mode:
@@ -218,15 +223,10 @@ def test_openpty(self):
         s2 = _readline(master_fd)
         self.assertEqual(b'For my pet fish, Eric.\n', normalize_output(s2))
 
-        os.close(slave_fd)
-        # closing master_fd can raise a SIGHUP if the process is
-        # the session leader: we installed a SIGHUP signal handler
-        # to ignore this signal.
-        os.close(master_fd)
-
     def test_fork(self):
         debug("calling pty.fork()")
         pid, master_fd = pty.fork()
+        self.addCleanup(os.close, master_fd)
         if pid == pty.CHILD:
             # stdout should be connected to a tty.
             if not os.isatty(1):
@@ -305,13 +305,14 @@ def test_fork(self):
             ##else:
             ##    raise TestFailed("Read from master_fd did not raise exception")
 
-        os.close(master_fd)
-
     def test_master_read(self):
+        # XXX(nnorwitz):  this test leaks fds when there is an error.
         debug("Calling pty.openpty()")
         master_fd, slave_fd = pty.openpty()
         debug(f"Got master_fd '{master_fd}', slave_fd '{slave_fd}'")
 
+        self.addCleanup(os.close, master_fd)
+
         debug("Closing slave_fd")
         os.close(slave_fd)
 
@@ -321,7 +322,6 @@ def test_master_read(self):
         except OSError: # Linux
             data = b""
 
-        os.close(master_fd)
         self.assertEqual(data, b"")
 
 class SmallPtyTests(unittest.TestCase):