Do support using pyvenv for python3

[MarSoft]

This PR closes #2

[randomir]

A couple of things that need to be done before merging:

• pyvenv doesn't have activate_this.py in env/bin/. Currently Envie relies on the existence of that file when searching for environments. This is easy to solve, but we also use that script for activation of the closest environment with import envie.activate_closest.

• does ensurepip has to be manually installed? On the first run of python3 -m venv I was required to install python3-venv package.

• we should have another battery of tests that cover pyvenv-created envs (~all existing tests but also for pyvenv)