[3.13] gh-138479: Ensure that `__typing_subst__` returns a tuple (GH-138482) (GH-138786)

* gh-138479: Ensure that `__typing_subst__` returns a tuple (GH-138482)

Raise an exception if __typing_subst__ returns a non-tuple object.

(cherry picked from commit 1da989be74eaa94590ec28e750e5352de1ead227)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>