parsers: adjust pure-python version to mimic a3dacabd476b

Authored by durin42.

Description

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

Details