tests: load showstack in test-pull-http.t so network hangs are easier to find
This also gives us some minimal "it loads" coverage on showstack,
which I rather like. showstack doesn't work on Windows per mbarbison,
so it's disabled there.
I added this in service of debugging a hang introduced on Python 3 by
revision a88d68dc3ee8. I'm still not sure what the problem there is,
but this at least gives us a little bit of a chance to figure out
what's going on.
Differential Revision: https://phab.mercurial-scm.org/D3247
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)