Gather interpreter constraints from all dependencies.
Review Request #519 — Created June 14, 2014 and submitted
Not just from the python_binary target itself. This allows us to put the constraints where they belong - on the code that requires them. Committed as 00a8f7ffec2cf41c7247f62845da6f288c457eab.
All unit/integration tests pass. Added a test for this functionality (plus, the existing tests now also exercise it). CI is ci-ing.