py3: write out hgextindex as bytes in setup.py
I hit this trying to build the py2exe target using python3, just to see what
would happen. After commenting out `py2exe.Distribution` in setup.py and
pointing to a local copy of py2exe that supports python3[1], it complained that
`out` was bytes, not str.
[1] https://github.com/albertosottile/py2exe/releases/tag/v0.9.3.0
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