[issue18299] Change script_helper to use universal_newlines=True in _assert_python

[hai shi]

hai shi <shihai1991 at 126.com> added the comment:

>it's a pain to extend the API (I don't want to use yet another __xxx custom keyword)
Adding __xxx in run_python_until_end function would increase the complexity but it looks like a unified function.

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nosy: +shihai1991

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue18299>
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