[issue30664] Change unittest's _SubTest to not sort its params when printing test failures

[Eric V. Smith]

Eric V. Smith added the comment:

Correct on the order changed with regular dicts. That's why I'm targeting this specifically for Python 3.7 and with **kwargs, where order is guaranteed. We might have to use a structure other than a ChainMap of dicts, like a ChainMap of OrderDicts.

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<http://bugs.python.org/issue30664>
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