Default to Python 3

[nailor]

Create Python 3 virtualenvs by default, with option to use Python 2.

Rationale: Python 2.7 will be EOL January 2020, so there's no point to have it as the default. While at it, might make sense to revisit internals of dh-virtualenv and see if they should be converted to Python 3 (or at least made compatible with it, if possible).

This change would be backwards incompatible