py3: make contrib/debugshell.py work with Python 3
I changed default mercurial installation of my personal laptop to one installed
with python 3.7. debugshell is one of the extension which I have enabled and it
was failing. This patch makes debugshell works with Python 3.
I found that chg does not work with python 3.
Differential Revision: https://phab.mercurial-scm.org/D6031