fuzz: add support for fuzzing under either Python 2 or 3 This was more of a hairball than I hoped, but it appears to work. The hg-py3 branch of my oss-fuzz fork on github has the remaining changes to switch us to Python 3, but we may as well retain Python 2 fuzzing support for at least a little while. Differential Revision: https://phab.mercurial-scm.org/D7592