[issue27458] Allow subtypes of unicode/str to hit the optimized unicode_concatenate block

[Raymond Hettinger]

Raymond Hettinger added the comment:

We really don't want to encourage any reliance on this optimization.  It was put there only to help mitigate the performance impact of a common mistake.

I don't think subclasses warrant extended coverage.  This was already a hack at the outer limits of how far we were willing to go.   It doesn't make sense to propagate it further.

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nosy: +rhettinger

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<http://bugs.python.org/issue27458>
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