[issue1564] The set implementation should special-case PyUnicode instead of PyString

[Raymond Hettinger]

New submission from Raymond Hettinger:

Much of the code in setobject.c exactly parallels that in dictobject.c.
 Ideally, we should keep that parallelism by scanning all of the changes
to dictobject.c and applying substantially similar changes to
setobject.c (just the changes that touch the hash tables, not the API
changes).

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nosy: +rhettinger

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