bpo-36346: Emit DeprecationWarning for PyArg_Parse() with 'u' or 'Z'. (GH-20927)

Emit DeprecationWarning when PyArg_Parse*() is called with 'u', 'Z' format.

See PEP 623.
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index df8f2c9..2626be6 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -2368,12 +2368,14 @@ def test_resize(self):
             text = 'a' * length + 'b'
 
             # fill wstr internal field
-            abc = getargs_u(text)
+            with self.assertWarns(DeprecationWarning):
+                abc = getargs_u(text)
             self.assertEqual(abc, text)
 
             # resize text: wstr field must be cleared and then recomputed
             text += 'c'
-            abcdef = getargs_u(text)
+            with self.assertWarns(DeprecationWarning):
+                abcdef = getargs_u(text)
             self.assertNotEqual(abc, abcdef)
             self.assertEqual(abcdef, text)