devtools: make github-merge.py use py3

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 it's not used by the build system - but it was easy.