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
 
@@ -250,6 +251,8 @@ class PlatformTest(unittest.TestCase):
             command = '"{}" -c "print(\'Hello\')"'.format(sys.executable)
         else:
             command = "'{}' -c 'print(\"Hello\")'".format(sys.executable)
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", DeprecationWarning)
             with platform.popen(command) as stdout:
                 hello = stdout.read().strip()
                 stdout.close()
@@ -261,6 +264,8 @@ class PlatformTest(unittest.TestCase):
         else:
             command = "'{}' -c 'import sys; data=sys.stdin.read(); exit(len(data))'"
         command = command.format(sys.executable)
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", DeprecationWarning)
             with platform.popen(command, 'w') as stdin:
                 stdout = stdin.write(data)
                 ret = stdin.close()