This makes github-merge.py the first developer tool to go all Python 3 (for context see #7717).
The changes are straightforward as the script already was from __future__ import division,print_function,unicode_literals.
However urllib2 changed name, and json will only accept unicode data not bytes.
This retains py2 compatibility for now: not strictly necessary as the script is not used by the build system (see #7723) - but it was easy.