bpo-33262: Deprecate passing None for `s` to shlex.split() (GH-6514)

* bpo-33262: Deprecate passing None for `s` to shlex.split()

This reads the string to split from standard input.

* Update What's New.

* Fix shlex.rst
diff --git a/Lib/test/test_shlex.py b/Lib/test/test_shlex.py
index a21ccd2..3081a78 100644
--- a/Lib/test/test_shlex.py
+++ b/Lib/test/test_shlex.py
@@ -3,7 +3,7 @@
 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 @@
             tok = lex.get_token()
         return ret
 
+    @mock.patch('sys.stdin', io.StringIO())
+    def testSplitNoneDeprecation(self):
+        with self.assertWarns(DeprecationWarning):
+            shlex.split(None)
+
     def testSplitPosix(self):
         """Test data splitting with posix parser"""
         self.splitTest(self.posix_data, comments=True)