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
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