bpo-36346: Prepare for removing the legacy Unicode C API (AC only). (GH-21223)

diff --git a/Include/cpython/unicodeobject.h b/Include/cpython/unicodeobject.h
index 88a97a4..0f19b2a 100644
--- a/Include/cpython/unicodeobject.h
+++ b/Include/cpython/unicodeobject.h
@@ -11,6 +11,8 @@
 
 /* --- Internal Unicode Operations ---------------------------------------- */
 
+#define USE_UNICODE_WCHAR_CACHE 1
+
 /* Since splitting on whitespace is an important use case, and
    whitespace in most situations is solely ASCII whitespace, we
    optimize for the common case by using a quick look-up table
@@ -1169,4 +1171,7 @@
    and where the hash values are equal (i.e. a very probable match) */
 PyAPI_FUNC(int) _PyUnicode_EQ(PyObject *, PyObject *);
 
+PyAPI_FUNC(int) _PyUnicode_WideCharString_Converter(PyObject *, void *);
+PyAPI_FUNC(int) _PyUnicode_WideCharString_Opt_Converter(PyObject *, void *);
+
 PyAPI_FUNC(Py_ssize_t) _PyUnicode_ScanIdentifier(PyObject *);