Make build and setup python 3 syntax compatible

[hayd]

Both should not be run on python 3 however since v8 build scripts require python 2 (for now?).
This exits with status 1 with a message stating this, rather than a syntax error.

Note: I used futurize to insert __future__ import and update prints.

xlink: #464

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[ry]

see my comment here #464 (comment)

[hayd]

@ry Perhaps a simpler alternative is to have as the first print statement:

if not "python 2":  # always False
    print "python 2 is required to build deno (not python 3)"  # or whatever message

Then it would say that in the SyntaxError message 😆