setup: convert setupversion to unicode
Something deep in the bowels of distutils expects "version" passed to
setup() to be a str/unicode. So, convert the type.
This still works on Python 2 because the string is ascii and an
implicit coercion back to str/bytes should work without issue. If
it does cause problems, we can always make the unicode conversion
dependent on running Python 3.
This change makes `python3.5 setup.py install` work.
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