Supported Python versions for an archive

[nir0s]

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Currently, the supported python versions are either provided via user input or automatically deduced (as a single version) from the python version currently used to create the wagon.

1. We should rethink allowing users to provide the versions. It might be irrelevant
2. We should redo the mechanism which understands the python versions supported as the current one is technically not true. We might want to iterate through all wheels and get the set of common versions and use them instead.

[nir0s]

We should also check how wheel implements this and directly use its implementation if possible.

[nir0s]

Looking at this, it seems like wheel's implementation is irrelevant here. Wheel's implementation simply takes the runtime env (ip, pp, py, etc..), appends the relevant python versions and uses it for a single wheel. The work to do here is to do cross-wheel version intersections. We'll leave it for 1.0.0 to focus on functionality first.

[nir0s]

Also, see here: http://wheel.readthedocs.io/en/latest/#defining-the-python-version