Attached is an example of how we could support simple named constants in __text_signature__.  The change to posixmodule.c is just a hack to exercise the code in inspect.Signature; I didn't have a good example handy.

When I apply the patch and run "x.py", I see:
    (path, *, dir_fd=None, follow_symlinks=True, fake=9223372036854775807)

This patch is just an example; I don't propose to check it in in this state.  (We'd still need to modify Argument Clinic to support these named constants, too.)