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.
Use 'utf-8' instead of the Python 2 default of 'ascii' to encode/decode
commit messages.
This can be removed when switching to Python 3, as 'utf-8' is the
default there.
Necessary for merging #7422 due to the ฿ in ฿tcDrak.
This is meant to be a direct translation of the bash script,
with the difference that it retrieves the PR title from github,
thus creating pull messages like:
Merge #12345: Expose transaction temperature over RPC
