unpack result._traceback without assuming its size

[LiangShe]

Resolve

ValueError: too many values to unpack

in _strip_unnecessary_contents_from_stack at for f, line_no, method, _ in result._traceback:

Issue #53 #56

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[LiangShe]

I signed it!

[shirishr]

I signed the CLA.

[junwang929]

I signed it!!