fix: port most of the way to python 3 Only most of the way because we now have to decide: if we want to keep the current .format() interface for the config in hgrc, we have to use unicodes to do formatting in Python 3, rather than bytes. I'm basically fine with that, so a follow-up patch will do so. Differential Revision: https://phab.mercurial-scm.org/D3300