Changed python3 support check script to print to `stderr`.
Changed python3 support check script to print to `stderr`.

The scripts this might be used in might use `stdout` to report
results we do not intend to interfere with, e.g.,
`support/apply-reviews.py` can print commands to execute in dry run
mode.

This patch makes the check script print to `stderr` instead of
`stdout`.

Review: https://reviews.apache.org/r/67348/

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