Fixed python3/post-reviews.py type error.
Fixed python3/post-reviews.py type error.

The main challenge when switching from Python 2 to Python 3 is
to handle the strings, as many of them are now byte strings. I
have checked the other uses of `sys.stdout.buffer.write` while
working on this commit to check that it was the last issue of
that type in python3/post-reviews.py.

Review: https://reviews.apache.org/r/67321/

Gilbert88/mesos,reneploetz/mesos,Gilbert88/mesos,andschwa/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,jpeach/mesos,kaysoky/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,andschwa/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,dforsyth/mesos,gsantovena/mesos,dforsyth/mesos,dforsyth/mesos,andschwa/mesos,andschwa/mesos,abudnik/mesos,gsantovena/mesos,abudnik/mesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,jpeach/mesos,abudnik/mesos,gsantovena/mesos,abudnik/mesos,reneploetz/mesos,andschwa/mesos,Gilbert88/mesos,Gilbert88/mesos,kaysoky/mesos,andschwa/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,gsantovena/mesos,gsantovena/mesos,jpeach/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,jpeach/mesos,andschwa/mesos