Update test_shlex.py to CPython 3.9

Lib/test/test_shlex.py

@@ -3,7 +3,7 @@ import itertools
 import shlex
 import string
 import unittest
-
+from unittest import mock
 
 
 # The original test data set was from shellwords, by Hartmut Goebel.
@@ -162,6 +162,11 @@ class ShlexTest(unittest.TestCase):
             tok = lex.get_token()
         return ret
 
+    @mock.patch('sys.stdin', io.StringIO())
+    def testSplitNoneDeprecation(self):
+        with self.assertWarns(DeprecationWarning):
+            shlex.split(None)
+
     # TODO: RUSTPYTHON
     @unittest.expectedFailure
     def testSplitPosix(self):