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

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

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(cherry picked from commit 1da989b)

Co-authored-by: Peter Bierma <[email protected]>
Co-authored-by: Serhiy Storchaka <[email protected]>

Author: ZeroIntensity