bpo-29240: readline now ignores the UTF-8 Mode (#5145)

Add new fuctions ignoring the UTF-8 mode:

* _Py_DecodeCurrentLocale()
* _Py_EncodeCurrentLocale()
* _PyUnicode_DecodeCurrentLocaleAndSize()
* _PyUnicode_EncodeCurrentLocale()

Modify the readline module to use these functions.

Re-enable test_readline.test_nonascii().
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 0274de6..576e7ad 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -1810,6 +1810,16 @@
     PyObject *unicode,
     const char *errors
     );
+
+PyAPI_FUNC(PyObject*) _PyUnicode_DecodeCurrentLocaleAndSize(
+    const char *str,
+    Py_ssize_t len,
+    const char *errors);
+
+PyAPI_FUNC(PyObject*) _PyUnicode_EncodeCurrentLocale(
+    PyObject *unicode,
+    const char *errors
+    );
 #endif
 
 /* --- File system encoding ---------------------------------------------- */