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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.