convert: add some utility code for working with shlex on Python 3 This could have gone in pycompat, but it's only needed in convert, so I figured it made more sense here. It's got py3 in the name and checks pycompat.ispy3, so we'll find it whenever we decide to drop Python 2 support in 20x6. # no-check-commit because of required foo_bar naming on the proxy class Differential Revision: https://phab.mercurial-scm.org/D2525