Fixed JSON object type error when using `support/apply-reviews.py`.
Fixed JSON object type error when using `support/apply-reviews.py`.

The error `TypeError: the JSON object must be str, not 'bytes'` was
visible when using `support/apply-reviews.py` in our CI. This issue
seems to only happen in old versions of Python 3 that are not supported
but the fix works with any version of Python 3 and allows us to not
change the Python interpreter used by reviewbot.

Review: https://reviews.apache.org/r/68970/

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