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
