test_platform: ignore DeprecationWarning on popen() test

Lib/test/test_platform.py

-import sys
 import os
-import unittest
 import platform
 import subprocess
+import sys
+import unittest
+import warnings
 
 from test import support
 
@@ -250,10 +251,12 @@ class PlatformTest(unittest.TestCase):
             command = '"{}" -c "print(\'Hello\')"'.format(sys.executable)
         else:
             command = "'{}' -c 'print(\"Hello\")'".format(sys.executable)
-        with platform.popen(command) as stdout:
-            hello = stdout.read().strip()
-            stdout.close()
-            self.assertEqual(hello, "Hello")
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", DeprecationWarning)
+            with platform.popen(command) as stdout:
+                hello = stdout.read().strip()
+                stdout.close()
+                self.assertEqual(hello, "Hello")
 
         data = 'plop'
         if mswindows:
@@ -261,15 +264,17 @@ class PlatformTest(unittest.TestCase):
         else:
             command = "'{}' -c 'import sys; data=sys.stdin.read(); exit(len(data))'"
         command = command.format(sys.executable)
-        with platform.popen(command, 'w') as stdin:
-            stdout = stdin.write(data)
-            ret = stdin.close()
-            self.assertIsNotNone(ret)
-            if os.name == 'nt':
-                returncode = ret
-            else:
-                returncode = ret >> 8
-            self.assertEqual(returncode, len(data))
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", DeprecationWarning)
+            with platform.popen(command, 'w') as stdin:
+                stdout = stdin.write(data)
+                ret = stdin.close()
+                self.assertIsNotNone(ret)
+                if os.name == 'nt':
+                    returncode = ret
+                else:
+                    returncode = ret >> 8
+                self.assertEqual(returncode, len(data))
 
 
 def test_main():