Ensure that we can support Python required versions

[pombredanne]

We must support the provided --python-version from the dad CLI.
Also some packages require a minimum version of Python with "python_requires". We should support this as part of the dependency resolution

[pombredanne]

This has been completed and merged in part: we support command line python version input but we do not support the "python_requires" yet. This second part is fairly more involved. I would like to push this to another milestone for the remainder as this requires additional design.