Make PyUnicode_Copy() private => _PyUnicode_Copy()

Undocument the function.

Make also decode_utf8_errors() as private (static).
diff --git a/Objects/codeobject.c b/Objects/codeobject.c
index c5057bd..550e284 100644
--- a/Objects/codeobject.c
+++ b/Objects/codeobject.c
@@ -249,7 +249,7 @@
             return NULL;
         }
         else {
-            item = PyUnicode_Copy(item);
+            item = _PyUnicode_Copy(item);
             if (item == NULL) {
                 Py_DECREF(newtuple);
                 return NULL;