Merge branch 'fix-virtualenv' into 'master'

Prefer pyvenv instead of virtualenv for Python 3.3+

`virtualenv` uses python2 by default on my machine.
Using `pyvenv` (see https://docs.python.org/3/library/venv.html#module-venv) instead of `virtualenv` fixes the problem and let's me perform `python3 setup.py install` in the virtual environment.

See merge request !119