stablerange: introduce an 'basicstablerange' This class hold all the basic of stable range but the part specific to the sorting. This allow to clearly express what is expected from each method and to generate tests output to be validated against the more complex implementation. While writing this, we spotted a bug in the current stable range implementation. Fix coming.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)