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