Fix T85573: Building with Python 3.10a5 fails

Replace deprecated _PyUnicode_AsString{AndSize} usage.

T83626 still needs to be resolved before 3.10 is usable.

source/blender/python/intern/bpy_library_load.c

@@ -361,7 +361,7 @@ static PyObject *bpy_lib_exit(BPy_Library *self, PyObject *UNUSED(args))
           for (i = 0; i < size; i++) {
             PyObject *item_src = PyList_GET_ITEM(ls, i);
             PyObject *item_dst; /* must be set below */
-            const char *item_idname = _PyUnicode_AsString(item_src);
+            const char *item_idname = PyUnicode_AsUTF8(item_src);
 
             // printf("  %s\n", item_idname);