Make _PyUnicode_TranslateCharmap() symbol private

unicodeobject.h exposes PyUnicode_TranslateCharmap() and PyUnicode_Translate().
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index df3a1b5..93c4ad9 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -8683,7 +8683,7 @@
     return res;
 }
 
-PyObject *
+static PyObject *
 _PyUnicode_TranslateCharmap(PyObject *input,
                             PyObject *mapping,
                             const char *errors)