Explicitly close a file to stop raising a ResourceWarning.

Lib/test/test_sysconfig.py

@@ -306,12 +306,13 @@ class TestSysConfig(unittest.TestCase):
         env = os.environ.copy()
         env['MACOSX_DEPLOYMENT_TARGET'] = '10.1'
 
+        with open('/dev/null') as dev_null:
             p = subprocess.Popen([
                     sys.executable, '-c',
                     'import sysconfig; print(sysconfig.get_platform())',
                 ],
                 stdout=subprocess.PIPE,
-            stderr=open('/dev/null'),
+                stderr=dev_null,
                 env=env)
             test_platform = p.communicate()[0].strip()
             test_platform = test_platform.decode('utf-8')