parsers: adjust pure-python version to mimic a3dacabd476b This was caught by the tests, to my surprise. I'll also follow up with a test of the index[-1] behavior so we can be sure that remains consistent, as I think that currently has no coverage. Differential Revision: https://phab.mercurial-scm.org/D4183

tests: restore Python 3 compat in test-parseindex2.py I think this was previously getting lucky in Python 3, and would have stacktraced if something failed. Now that failure messages are eagerly computed, this turned into a problem. Differential Revision: https://phab.mercurial-scm.org/D4182

tests: port remaining bits of test-parseindex2 to unittest asserts Differential Revision: https://phab.mercurial-scm.org/D4181

tests: allow more detailed error message from re.compile test-grep.t now passes on Python 3. Differential Revision: https://phab.mercurial-scm.org/D4240