Use PyEnv Virtualenv plugin

[archena]

Is your feature request related to a problem? Please describe.
Using venv separately from PyEnv causes interoperability problems.

Describe the solution you'd like
Since we use PyEnv to manage Python, we should consider using the Virtualenv plugin for Pyenv as well. The plugin: https://github.com/pyenv/pyenv-virtualenv

This needs implementing and, crucially, testing on multiple operating systems.