bpo-36346: Raise DeprecationWarning in deprecated usages.

Raise DeprecationWarning when:

* PyArg_ParseTuple with 'u', 'Z' format.
* PyUnicode_FromStringAndSize(NULL, size)

We can not use C level deprecation because these API will be not
removed.

Author: methane

Misc/NEWS.d/next/Core and Builtins/2021-02-22-19-00-00.bpo-36346.uAoni0.rst

@@ -0,0 +1,2 @@
+``PyArg_Parse*()`` functions now emits ``DeprecationWarning`` when ``u`` or
+``Z`` format is used.

Python/getargs.c

@@ -1014,7 +1014,10 @@ convertsimple(PyObject *arg, const char **p_format, va_list *p_va, int flags,
     case 'u': /* raw unicode buffer (Py_UNICODE *) */
     case 'Z': /* raw unicode buffer or None */
     {
-        // TODO: Raise DeprecationWarning
+        if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
+                "getargs: The '%c' format is deprecated. Use 'U' instead.", c)) {
+            return NULL;
+        }
 _Py_COMP_DIAG_PUSH
 _Py_COMP_DIAG_IGNORE_DEPR_DECLS
         Py_UNICODE **p = va_arg(*p_va, Py_UNICODE **);