Use venv module in python 3

[rpkilby]

This is a proof of concept for issue #436. The fix is incoming after the failing test run is complete. I'm assuming this will probably fail on python 3.3, since the --copies option wasn't implemented yet.

[rpkilby]

I've pushed the code to demonstrate that migrating to venv shouldn't be too difficult. Either way, closing, as the original cause was not directly related to warnings.