move ensure_python() from setup.py into Kwant and use it

This allows to provide a helpful error message when someone tries to
import Kwant with Python 2.

Note that I moved the version code into a module of its own:
kwant.version.  The rationale is to separate this code from unrelated
stuff in kwant._common that requires various external libs.  The
kwant.version module should work on its own without any third-party
libraries.