posix.confname(): enforce that names as str instances.

Modules/posixmodule.c

@@ -5455,11 +5455,15 @@ conv_confname(PyObject *arg, int *valuep, struct constdef *table,
         int cmp;
         const char *confname;
         Py_ssize_t namelen;
-        if (PyObject_AsCharBuffer(arg, &confname, &namelen) < 0) {
+        if (!PyUnicode_Check(arg)) {
             PyErr_SetString(PyExc_TypeError,
                             "configuration names must be strings or integers");
             return 0;
         }
+        confname = PyUnicode_AsString(arg);
+        if (confname == NULL)
+            return 0;
+        namelen = strlen(confname);
         while (lo < hi) {
             mid = (lo + hi) / 2;
             cmp = strcmp(confname, table[mid].name);