Skip the test requiring ctypes if ctypes is unavailable.

prevents http://buildbot.python.org/all/builders/x86%20Ubuntu%20Shared%203.x/builds/240/steps/test/logs/stdio

Lib/test/test_subprocess.py

@@ -4,7 +4,6 @@
 import subprocess
 import sys
 import platform
-import ctypes
 import signal
 import io
 import os
@@ -18,6 +17,11 @@
 import gc
 import textwrap
 
+try:
+    import ctypes
+except ImportError:
+    ctypes = None
+
 try:
     import threading
 except ImportError:
@@ -2491,6 +2495,7 @@ def test_communicate_BrokenPipeError_stdin_close_with_timeout(self):
       'Linux': 'so.6',
       'Darwin': 'dylib',
     }
+    @unittest.skipIf(not ctypes, 'ctypes module required.')
     @unittest.skipIf(platform.uname()[0] not in _libc_file_extensions,
                      'Test requires a libc this code can load with ctypes.')
     @unittest.skipIf(not sys.executable, 'Test requires sys.executable.')