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.

setup.py

@@ -661,7 +661,14 b" for root in ('templates',):"
             packagedata['mercurial'].append(f)
 
 datafiles = []
-setupversion = version
+
+# distutils expects version to be str/unicode. Converting it to
+# unicode on Python 2 still works because it won't contain any
+# non-ascii bytes and will be implicitly converted back to bytes
+# when operated on.
+assert isinstance(version, bytes)
+setupversion = version.decode('ascii')
+
 extra = {}
 
 if py2exeloaded: