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

setup.py

@@ -583,9 +583,9 b' class buildhgextindex(Command):'
         if err or returncode != 0:
             raise DistutilsExecError(err)
 
-        with open(self._indexfilename, 'w') as f:
-            f.write('# this file is autogenerated by setup.py\n')
-            f.write('docs = ')
+        with open(self._indexfilename, 'wb') as f:
+            f.write(b'# this file is autogenerated by setup.py\n')
+            f.write(b'docs = ')
             f.write(out)
 
 class buildhgexe(build_ext):