requirements: Added "numpy" as a requirement

[villesundell]

Added numpy (fundamental package for scientific computing with Python) as a requirement (run-time).

[konradkonrad]

This seems invalid at first sight, but I'll give you the chance to explain the rationale behind this PR, @villesundell ?

[villesundell]

Sorry, I think this is only for "serenity"-branch (the test file in ethereum/ needs this). My bad, I can make another PR against "serenity" when I return to my computer.

[villesundell]

Nope, it seems that "develop" branch also uses numpy at: ethereum/fastvm.py
Am I mistaken/misunderstood something here?

[konradkonrad]

Hmm. To my knowledge fastvm.py is not in use, but so far just a WIP/PoC. So I'm hesitant to add numpy to the global requirements.
How did you end up with this PR? Have you successfully used fastvm.py for anything?

[villesundell]

Sorry for the delay! Well, some of Vitalik's Serenity PoC instructions used test.py, as in here: https://blog.ethereum.org/2015/12/24/understanding-serenity-part-i-abstraction/