reassess actual dependencies

[robertmuil]

Currently, requirements.txt is a bit over-zealous with versions: expan doesn't actually need the latest version (0.17) of scipy for example.
To increase backward compatibility, we should go over what's actually required. Potentially long-winded operation I guess but perhaps there's a tool to do it semi-automatically?

[gbordyugov]

I'm not aware of that kind of tools, would be glad to learn about them.

[jbao]

@gbordyugov The current idea is to automatically generate a bunch of requirements.txt with different combinations of library versions, and do some kind of grid search to check the dependency.